This game is addictive.
Most people would cite that as a positive, but in this case it is not.
Chances are, if you've been around this game for a little while, you would have heard about the grind. While this true, I'm not the sort of person do avoid grind (hell, I've been playing the Monster Hunter and Dragon Quest franchise for years). The problem comes once grind stops being about working towards a goal and instead tries too replace the goal. Grind is not content. Grind is not fun. People grind so they can get a payoff from their hard work by annihilating the BBEG in two turns. Grind is meant to pad out game time yes, but it is also made to enhance the power fantasy. The grind within Warframe does not

Warframe is a game that has been built up over years. As impressive as this is, it is also it's biggest flaw.
Because of it's continuous (albeit slow) stream of content and reworks, many aspects of the game lag behind in the design, and in some cases are completely forgotten or ignored (Conclave, Lunaro, etc). All this builds up to a point where they are talking about making an open world for each planet but at the same time almost 50% of the mods in the game are utterly useless. Mods are essentially the basis on which the current combat system is built upon. Combat is the main form of gameplay. If you've got holes in one of your main mechanics, it should be on the top of your list to fix. Otherwise you risk the while thing becoming top-heavy and collapsing. Now I used the mods system here, but this can be applied to many other areas of the game that are vital for it to function, like the AI, or the fact that the game is basically split up into islands of content that have almost nothing to do with each other.

The introduction of the Plains of Eidolon and Fortuna updates were pretty big red flags. Nothing of that sort had ever existed in the game beforehand. This means that, due to DE not having ample experience with open worlds, both launched in an unbearably buggy state with an obscene grind for very little reward. However, the main issue with these updates was not the grind or rewards, the issue was it showed just how starry eyed DE had become. They may be an independent studio, but they seem to think of themselves as a large corporation.

Now, As you can tell, I have spent quite some time playing this game (1.3K hours as of writing), and you might ask; "It took you that long to notice? This dude sounds entitled". This is a fair point, but I opened this review saying this game is addictive did I not? I have burned out on this game many times, and I keep coming back. Why? It's not like I don't have a huge backlog of games that I could be playing.
I don't like to admit it, but it's because I'm addicted to it. Not to grind, weapons, warframes or even mission types. What I love about this game is the movement. Warframe has one of the most versatile and gratifying movement systems I have ever come across. I chose to almost exclusively use melee because it requires you to move more. If there is one compliment I would give DE it's that they absolutely nailed the parkour system.

I'll wrap up this review as I feel I'm getting off track.
In short, Warframe is a very impressive product that sells the power fantasy of being a sci-fi weapon of mass destruction better than any game I've played, but that novelty does wear off, and when it does you will feel as though you have wasted much of your time chasing the next meta.
Grind for the sake of progress is satisfying.
Grind for the sake of grind is not satisfying, it's unpaid labor.