The most common gripe I hear is the ship controls. It’s really designed for controller, and manual space flight is kind of pointless when the autopilot can do it so much better (as long as there’s nothing in the way).
It’s really a game about pulling threads until you find interesting things. The ship’s computer is useful for keeping track of the thread ends. Most places can be reached in a minute or so right from the main planet, although depending on where you’re going you might have to wait longer to get access to what you want. There isn’t really any “starting over” because what you’re gathering is knowledge in your own head.
I think the hardest part initially is finding an interesting thread to pull on. I think Giant’s Deep was where it started to click for me, but there’s undoubtedly more.
I think Unity’s main issue was they marketed the multiplayer heavily, but it was only in the game for an extremely limited number of missions.
They managed to annoy single player gamers and multiplayer gamers in one stroke.
They got overambitious with the crowds as well, leading to poor performance on consoles, along with a whole load of weird bugs. I played it on PC years later (maybe when the Notre Dame burned down?) and there were still parts of the game bugged, or chugged down to 20 fps for no reason.
I don’t hate them as games, but the need for gear to be able to actually do assassinations on the bigger guys somewhat makes it not an AC game any more. Still the same slightly tired gameplay, but the one element that made it satisfying is no longer there.