I'm curious about the difficulty curve. As long as the puzzles take the form "group of left-facing frogs - empty pad - group of right-facing frogs", they'll all have exactly the same solution. If you can do level 2, you can do level 2222 without needing to come up with any new ideas.

This kind of blocks the notion of a "difficulty curve" - it's just a flat line.

Level 4 is different, but I'm guessing the reason people say it's easier is that the frogs on the left never need to interact in any way with the frogs on the right. You can just march your left-frogs onto the transformer, see them turn around, and march them back to their ultimate destination, then repeat the process on the right.

I don't think there's any solution that would allow a frog to jump over the transformer, so you're essentially required to do this - all frogs must make their way to the transformer, transform, and go back home - although you can make it look more or less complex. This essentially gives you two copies of the "level 3 and below" puzzle, one on the left of the transformer and the other on the right.