I found 25->50 hard my first time, and 100-200 hard my first three times

But don't blow skill differences out of proportion between the limits. It's the same game. Table select well and you can find 100NL tables easier than some 10NL tables. It's just that on average they're not...

The mental hurdles of dealing with the "bigger money" is the biggest thing you've gotta overcome when moving up. Not changing your game is by far the more important thing, and it's hard for a lot of people when they assume the game will be drastically different.