TL;DR: I enjoyed most of time with the content of Sunless Skies...but there's so much filler that I wouldn't recommend it. I think Sunless Sea is a better experience overall, for several reasons.

But first, things I liked. The writing is good, as always for Fallen London stuff. The imagery in Skies is beautiful, and more visually conveys both the eldritch horrors of the world and the authoritarian methods humanity has to use to conquer them. The world is much more constant in comparison to Sea; my first run was spent mostly upgrading officers so that my later lineages would have better options. I overall prefer the combat in Skies to Sea, as there's a lot more nuance to the types of weapons available. Heat is also an interesting idea of a resource, but it has some flaws I'll get to. Finally, I like the levelling system in theory when compared to Sea, because it makes a lot of the harder choices more permanent. The Secrets of Sea were infinitely farmable; levels are not.

As for problems...

Sea's difficulty all came in the first 10 hours or so, where you lacked knowledge and resources to move much beyond London. Once you figured out the echo farming loops and where to get all the resources for quests, it was just a matter of putting it all together. Skies' difficulty curve is much less flat, but for all the wrong reasons: Terror is an actual issue in Skies, since it both accumulates faster and is significantly harder to get rid of; Nightmares have a tendency to lose you stats permanently, and are ridiculous to remove; and your resources are much more limited (12 cargo in the starting ship compared to 40). None of these would inherently make the game not recommendable, but there are some design issues that Sea doesn't have at all.

For example, it's impossible to guarantee a good result on a lot of checks due to a ♥♥♥♥♥♥♥♥ 'partial success' roll in the background that only activates if you would've normally succeeded. (Don't trust the wiki, either; it is NOT just 5%.) Skies also has a ton of trap options: don't ever run a Secondment with one of your officers unless their upgrade is redundant to another officer in their station. A lot of weapon upgrades don't meaningfully increase your DPS because the Heat bar never upgrades, and bigger weapons often generate more heat. You also have to aim with most weapons, so fewer shots means you need to be more accurate. Another one is engine upgrades: the math is complicated, but in short, they all cover less distance per unit of fuel than your starter engine, and at best increase your speed by 70%. If you want to go fast economically, you're forced to ♥♥♥♥♥♥♥ dash everywhere. (These engine upgrade also didn't even exist until the Sovereign patch launched.)

But why does speed matter so much that they'd make highly inefficient engines just to move faster? Well, despite the High Wilderness being obsensibly outer space, there are a lot of rocks and walls in the way, so going as the crow flies isn't possible in 95% of situations like it was in Sea. Even then, I reinstalled Sea to do the comparison...

• in Sea, with the basic engine, ship, and no Full Steam Ahead, it took about 11 minutes to go from the Dawn Machine to Irem in a straight line, i.e. one corner to the opposite.
  
• in Skies, from London to the one edge of the circle I could reach in a straight line under the same conditions, took ~3.5 minutes, so 7 minutes for the full diameter if there weren't walls in the way...

But there are also four different areas, with many of the access points being potentially opposites from each other. Movement between ports is glacial in comparison to Sea. You can't even set a course and look away at Youtube or something because of how often you need to turn!

You wanna know how bad it is? The universally recommended method to not make this game be a slog is to install Cheat Engine, and enable a hotkey to turn on and off the speedhack. After about 10 hours of the vanilla speed, I took that option and put the modifier to 2.5x, turning it off only for combat. If I want to compare completion times, Sea + Zubmariner took me 168.9 hours to 100%, and that has 7 ambitions and some abnoxious achievements (like spending ~61 hours actively at sea). While my real time for Skies is above, the adjusted time for the speedhax is ~159 hours (reducing the multiplier to x2 to take into account disabling it for combat encounters.) For a game with less content.

To cap it all off, I've only played the Sovereign patch so I don't know what changed, but it seems like some of these complaints stem from changes made in said patch. I've seen posts online of people saying that Terror was easier to manage here than it was in Sea, for example.

TL;DR: I enjoyed most of time with the content of Sunless Skies...but there's so much filler that I wouldn't recommend it. I think Sunless Sea is a better experience overall, for several reasons. But first, things I liked. The writing is good, as always for Fallen London stuff. The imagery in Skies is beautiful, and more visually conveys both the eldritch horrors of the world and the authoritarian methods humanity has to use to conquer them. The world is much more constant in comparison to Sea; my first run was spent mostly upgrading officers so that my later lineages would have better options. I overall prefer the combat in Skies to Sea, as there's a lot more nuance to the types of weapons available. Heat is also an interesting idea of a resource, but it has some flaws I'll get to. Finally, I like the levelling system in theory when compared to Sea, because it makes a lot of the harder choices more permanent. The Secrets of Sea were infinitely farmable; levels are not. As for problems... Sea's difficulty all came in the first 10 hours or so, where you lacked knowledge and resources to move much beyond London. Once you figured out the echo farming loops and where to get all the resources for quests, it was just a matter of putting it all together. Skies' difficulty curve is much less flat, but for all the wrong reasons: Terror is an actual issue in Skies, since it both accumulates faster and is significantly harder to get rid of; Nightmares have a tendency to lose you stats permanently, and are ridiculous to remove; and your resources are much more limited (12 cargo in the starting ship compared to 40). None of these would inherently make the game not recommendable, but there are some design issues that Sea doesn't have at all. For example, it's impossible to guarantee a good result on a lot of checks due to a bullshit 'partial success' roll in the background that [i]only activates if you would've normally succeeded. [/i] (Don't trust the wiki, either; it is NOT just 5%.) Skies also has a ton of trap options: don't [b]ever[/b] run a Secondment with one of your officers unless their upgrade is redundant to another officer in their station. A lot of weapon upgrades don't meaningfully increase your DPS because the Heat bar never upgrades, and bigger weapons often generate more heat. You also have to aim with most weapons, so fewer shots means you need to be more accurate. Another one is engine upgrades: the math is complicated, but in short, they all cover less distance per unit of fuel than your starter engine, and [i][b]at best[/b][/i] increase your speed by 70%. If you want to go fast economically, you're forced to fucking [i]dash[/i] everywhere. (These engine upgrade also didn't even exist until the Sovereign patch launched.) But why does speed matter so much that they'd make highly inefficient engines just to move faster? Well, despite the High Wilderness being obsensibly outer space, there are a [i]lot[/i] of rocks and walls in the way, so going as the crow flies isn't possible in 95% of situations like it was in Sea. Even then, I reinstalled Sea to do the comparison...[list] [*]in Sea, with the basic engine, ship, and no Full Steam Ahead, it took about 11 minutes to go from the Dawn Machine to Irem in a straight line, i.e. one corner to the opposite. [*]in Skies, from London to the one edge of the circle I could reach in a straight line under the same conditions, took ~3.5 minutes, so 7 minutes for the full diameter if there weren't walls in the way...[/list] But there are also four different areas, with many of the access points being potentially opposites from each other. Movement between ports is [i]glacial[/i] in comparison to Sea. You can't even set a course and look away at Youtube or something because of how often you need to turn! You wanna know how bad it is? The universally recommended method to not make this game be a slog is to install Cheat Engine, and enable a hotkey to turn on and off the speedhack. After about 10 hours of the vanilla speed, I took that option and put the modifier to 2.5x, turning it off only for combat. If I want to compare completion times, Sea + Zubmariner took me 168.9 hours to 100%, and that has 7 ambitions and some abnoxious achievements (like spending ~61 hours actively at sea). While my real time for Skies is above, the adjusted time for the speedhax is ~159 hours (reducing the multiplier to x2 to take into account disabling it for combat encounters.) For a game with less content. To cap it all off, I've only played the Sovereign patch so I don't know what changed, but it seems like some of these complaints stem from changes made in said patch. I've seen posts online of people saying that Terror was [i]easier[/i] to manage here than it was in Sea, for example.