It was kind of similar for me, initial impression was 5 stars which sunk to 3,5 stars mid-game when things started to feel repetitive and lacking in challenge. But as I continued, the game picked up considerably and missions started to become more challenging in the last third of the game. Only one mission felt a bit unfair, the one with the prison cells. But it's entirely doable, even if it takes 3-4 failed attempts. With missions being so short, it's not a big loss of gaming time. I've experienced far worse in other games.

I think giving 3 stars to this game is very clearly not enough. Subjectively, you can of course do that but objectively rated (based on logic and facts) that is unfair and unreasonable. Many times have I loudly cursed at games out of frustration but I would never take it out on the game if it's a matter of my skills failing rather than the game itself failing. There are instances where games are in fact broken, Crimsonland Remastered is such an example. I wrote a long review for that game explaining how the game is more luck than skill based. Even so, I didn't post the review to the store pages as my review included a lot of subjective anger and ranting as well as objective info.

The monsters hit shots as well, don't be y crybaby :P
Yes, it can be annoying but in this game position is key. Don't try to shoot at anything from too far away if a character doesn't have enough aim. 70% is good enough of a chance in situation where multiple characters can shoot but if you're in a hairy situation with only 2 characters one of which has sucky aim, then 70% chance to hit might be too low and you'd better look for a better position and only shoot over super far distances once Morten's steady aim becomes available.
I've never run out of ammo in this game, most of the time I carried way too much and on several occasions had to drop ammo because I found some valuable loot and had no inventory space. Despite bringing Sam along on every single mission where it was possible. I really like the extra space in his inventory, very useful. Still had to play inventory tetris between the crew members and that is annoying but at the end of the game I had over 5000 energy credits left so clearly I was a paranoid hoarder. Bringing 2-3 extra ammo boxes per person is usually more than enough unless you're using a gun with only 5 or less shots in which case you might run out. I didn't use such low capacity weapons a lot. Only the "Old Rifle" which iirc has about 4 shots but is still a very powerful sniper rifle. For the most part, I used sniper rifles with around 6 shots or more (later in the game). I only ever had to buy ammo in some of the later missions. For the finale, I didn't even bring much as I didn't know it was the last mission consisting of several parts so I only carried 2 ammo per person. But there is plenty of stuff in crates and I managed well enough. So ammo shortage isn't really a problem if you use position wisely and choose weapons with medium capacity (5-6+ shots) rather than low capacity (3-4 shots)

I'm not sure which mission you're referring to, perhaps you can check out Youtube for strategy guides? I'm sure there are several walkthroughs available by now if you just want to get past the mission and don't enjoy the challenge.

The mission design is repetitive but it's not stupid or "imba broken". There's always a way and often multiple ways to solve any situation. If you can bring Josh along , his shield removing ability is very useful. Followed up by a steady shot from Morten, that's a good combo. Sam's headshot as well, if you can bring him along. Don't get too pissed about Schaffer, I didn't even bother to stimpack that guy into anything particular, you could turn him into a sniper but imho that's a waste. I just used his reveal enemies skill (super useful in tight quarters) and gave him grenades & a good shield and used him as a crate runner and general mule, he can take a decent amount of damage due to his increased shield passive skill. Once he has 8 aim you can give him a railgun (more accurate then sniper for low aim people) but it's kind of a waste of ammo even then. He is mandatory on many missions and I never liked him much but my other characters were always skilled enough to deal with enemies even without Schaffer doing much violent stuff himself.

EDIT: retreat to a better position instead of shooting from too far. That's a good general recipe, works almost anytime. Don't give up too easily, everyone gets frustrated in certain spots!