I found there was not enough variety with the encounters (you keep seeing the same "abandoned space station"/"rogue drone" events very regularly). It's hard to forgive because all events are text-based, so it's not really difficult to add variety. That being said, maybe this extension solves this issue by adding a lot more events.

Others point out the extreme randomness of the game but I think it's only an issue because the game forces you to push forward (backtracking is almost never possible or very costly) which reduces your margin of error and makes you very dependent on the clemency of the "Random Number God". It's of course a very explicit design choice and the game wouldn't be the same without it but it does make for some very frustrating moments.

But I think the main problem I've had with the game is that it's sold as a roguelike (or rather, a roguelike-like) and in my opinion in doesn't come close to DCSS, nethack, angband and others in terms of depth of gameplay and "replayability". For me a roguelike is a game where you can end up in a seemingly desperate situation and still succeed by thinking out of the box and use an unexpected strategy. I've never had one of those moments in FTL. You can't take over other people's ships for instance, which would open a lot of new strategies. Of course the definition of "roguelike" is a bit loose so maybe I'm just too narrow minded.

All that being said for it's price it's definitely worth trying and see for yourself, I know some people have sunk hundreds of hours in this game so I guess they found something in it that eluded me...