OTOH when I played 100 Mario Challenge on Easy, it wasn't empty levels. It wasn't like, I want to endlessly do this to unlock costumes, but it was okay for unlocking a few costumes so I had some.
I'm enjoying the Normal level of difficulty for the 100 Mario Challenge. For each batch of 16 levels, I star about 4 of them.
One problem is that the game itself subtly pushes you into making short, gimmicky levels rather than longer, robust levels. There are no checkpoints, that's the biggest thing. The game is filled with small disappointments, though. There's no way to do the standard Mario thing where if you're small Mario, a block contains a mushroom, but it contains something else if you're big Mario. If you put a mushroom in a block, it's always a mushroom, and ditto with the other power-ups. You can go backwards in SMB1 levels. A lot of little stuff like that which adds up. There is a lot of neat stuff you can do, though. I will eventually get used to all the little changes.
Once I finally unlock invisible blocks, I will probably publish my first level. Any decent Mario level has invisible blocks.