So the vanilla game is kind of weird when it comes to difficulty because it has a reverse difficulty curve. The game starts out pretty unforgivably hard at first because your rookies just don't really have many options to deal with what is going on and it causes major issues in the campaign to the point that if you fail 1 or 2 missions early with either of them result in you losing rookies then you pretty much lose the campaign. This is amplified on legend. If you can make it to later game without sustaining too many loses, can get soldiers ranked up and give them new gear then the game starts becoming A LOT easier because there are no newer alien types that have abilities that are too hard to deal with. At max rank with max gear your soldiers have so many good skills that you can just steamroll through the game, even if you move the story along fast.

Long War is the opposite. The early game starts off pretty easy as your soldiers have plenty of options of dealing with the basic enemy types as all of your soldiers will have a slot for a flash bang and a grenade and a vest that lets you absorb 2 points of damage. With all of these options it is not hard to get a Flawless rating on a lot of the early game missions.

Later on in a Long War campaign the difficulty starts ramping up for many reasons. Enemies become stronger due to fighting new enemies and stronger versions of old enemies, dark events that affect the tactical layer are mostly all (if not all) permanent and when a lot of them stack up it starts to become a real problem (aliens with 3 additional hitpoints, higher aim, higher detection radius, different types of ammo like Viper Rounds). Also the Avatar Project does take longer to finish now that it has more bubbles to fill up than before but the rate at which it completes seems to be a little faster (at least to me that is). There are more retaliation types and they can happen quite often (I have had situations where there was 2 retaliations happening at the same time).

Another thing that makes long war harder is that when enemies call in reinforcements you cannot see the flare anymore (actually that is a lie, you can still see it on Rookie difficulty but veteran and up you cannot see if anymore) and when the reinforcements do come the game tries to place them in the most inconvenient place possible (they usually spawn in a place where you will be flanked or easily flankable unless you kill them right away, most of the time they will come in behind you when you are trying to move up on an objective).

All of these things add up to the difficulty ramping way up later in the game (some might argue a little too much) in long war. Hopefully this gives you an idea of what kind of difficulty to expect when at least playing Long War.

Originally posted by JM001:

So the vanilla game is kind of weird when it comes to difficulty because it has a reverse difficulty curve. The game starts out pretty unforgivably hard at first because your rookies just don't really have many options to deal with what is going on and it causes major issues in the campaign to the point that if you fail 1 or 2 missions early with either of them result in you losing rookies then you pretty much lose the campaign. This is amplified on legend. If you can make it to later game without sustaining too many loses, can get soldiers ranked up and give them new gear then the game starts becoming A LOT easier because there are no newer alien types that have abilities that are too hard to deal with. At max rank with max gear your soldiers have so many good skills that you can just steamroll through the game, even if you move the story along fast.

Long War is the opposite. The early game starts off pretty easy as your soldiers have plenty of options of dealing with the basic enemy types as all of your soldiers will have a slot for a flash bang and a grenade and a vest that lets you absorb 2 points of damage. With all of these options it is not hard to get a Flawless rating on a lot of the early game missions.

Later on in a Long War campaign the difficulty starts ramping up for many reasons. Enemies become stronger due to fighting new enemies and stronger versions of old enemies, dark events that affect the tactical layer are mostly all (if not all) permanent and when a lot of them stack up it starts to become a real problem (aliens with 3 additional hitpoints, higher aim, higher detection radius, different types of ammo like Viper Rounds). Also the Avatar Project does take longer to finish now that it has more bubbles to fill up than before but the rate at which it completes seems to be a little faster (at least to me that is). There are more retaliation types and they can happen quite often (I have had situations where there was 2 retaliations happening at the same time).

Another thing that makes long war harder is that when enemies call in reinforcements you cannot see the flare anymore (actually that is a lie, you can still see it on Rookie difficulty but veteran and up you cannot see if anymore) and when the reinforcements do come the game tries to place them in the most inconvenient place possible (they usually spawn in a place where you will be flanked or easily flankable unless you kill them right away, most of the time they will come in behind you when you are trying to move up on an objective).

All of these things add up to the difficulty ramping way up later in the game (some might argue a little too much) in long war. Hopefully this gives you an idea of what kind of difficulty to expect when at least playing Long War.

Thank you for your answer!

Great explenation in the post above , I must say.
To potentialy save you some frustration , start the game on recruit for the reasons that were stated .

I started on veteran , and about 35 missions in , had to quit . I didn't play flawlessly but there were maybe several mistakes like 5 soldiers dying , one liberated haved destroyed (I didn't have people for defense mission ) so it had no people and probably more mistakes BUT
I had 45 soldiers that were constantly working on missions/training and most were very successful .

Right now , I think that veteran is extremly hard for anyone that is not methodical player that takes a long time to act during tactical missions and use almost all tools at their disposal

My rule of thumb is that if average advent/alien that you are facing has about the same stats as your soldier , you've already lost and can restart

I actually searched for let's play videos on youtube and couldn't find anyone past maybe 50 missions and those that got there have extremly hard time . You can watch marbozir videos , he is very good player playing long war 2 on veteran and I think he will have to restart and he didn't make any major mistakes