Arcanum is an awful game, it has a good world, good side quests, but the game systems are completely broken, in a way that is very likely to impact the player. Not just a few specific builds are broken, but the entire system of building characters.
PS:T is a bad game with a good plot. Well not good so much as complicated and verbose, but it's a solid plot. Plot and dialog aside, the gameplay fails utterly to even come close to say BG1.
Fallout is a mediocre game (shallow combat, bad AI), but not an awful one. And the combat while mediocre was fresh at the time, combine that with the awesome animations and it seemed fun while you were playing the first time. Plus it had the best world out of the three, and was more original and innovative, so when you first played it, the flaws didn't stand out and you got sucked into it.
However, I must point out that BG2 probably literally provides more actual good gameplay than all 3 combined.