Just finished SwapBot's 12 levels (after Bubble Butler's 30). "Enjoyed" both a lot. Tough but satisfying. I presume I played the updated levels. The comments on Google's discussion page suggest there's been several iterations of the game thus far. For the record, I found levels 5 and 9 to be hardest. Others no picnic, but didn't take days to solve. Levels 8 and 10 seemed perfectly fine to me, though perhaps "easier" than others, probably because they're basically mazes. Many thanks to JMW.
Hey, thanks for playing; I'm glad you "enjoyed" them! Yeah, I think the difficulty in SwapBot is pretty front-loaded since a lot of the solutions require the player to discover things about the mechanics, and there's less to discover as you go on. 5 requires something very bizarre/unintuitive, and I think 9 just requires good swap optimization? 9 actually has a couple nearly distinct solutions, but many people find it to be one of the hardest despite that.
Good job on beating Bubble Butler, too! That game has more filler and is a little clunkier, but I think I still managed to make some satisfying puzzles. It's very heavily inspired by Little Magic (or at least my memory of Little Magic) so I think they're not quite as original. Still pretty proud of Level 29 though!