As you see the "holes" are designed in such a way that you cannot just put the key and that's it. You have to fist find the good hole, then put the first part of the key in the right position and then make some moves with the key to put it completely inside.
It did not take long time before solving each cube, so I would not say that it's as challenging as they mention (they are rated as 4/5) but they are pleasant to solve.
I do think this is a very great idea, it's like a one piece packing puzzle. I wish some more puzzles like these can be designed.
These puzzles are from this webshop, they have several interesting puzzles. Have a look!