you're
really letting nostalgia get in the way of your memory. UC2 approached arenas in the same exact way UC1 and UC3 do (through trial and error). in UC2 each area of battle a wave of AI would pour out, just like in UC3 and UC2. the only thing I will give you that UC2 did as opposed to its prequel or sequel was the amount of variety between those arena battles.
in UC1 you would go from arena, to arena to arena. in UC2 you would go from arena, to sneaking, to running through buildings as a helicopter chases you, to carrying a dying camera man as many mercs shoot you, to solving a platforming puzzle, to exploring icy caves with monsters, eh, you get the picture.
now UC3's pacing is all fucked up. this is the problem that many people seem to be having with it. UC2 has more arena battles than UC3 but the difference is that UC3 gives you the variety and unique combat scenarios at the beginning of the game while UC3 spams you with arena battles from mid-end until the end of the game with barely any room to breathe. I feel like many of the people who dislike UC3's arenas are experiencing fatigue from the horrible lack of pacing found in this game.