I always used the "doesn't work on my system". Though, most of the games I've refunded were really not working on Linux the way I'd like and I just didn't want to hack around or have to reboot into Windows for that game.
Don't forget the two weeks since purchase, which is especially nefarious as Steam banks on people buying many games through sales that they will only play much later.
Two hours is far more than enough to determine if a game is for you.