I've looked at these challenges and considered trying them out, but there's an opportunity cost involved in doing them.

I'm sure they're of some value, but I prefer to spend my time that I have for professional learning either reading books or articles, learning new tools or trying out new tools and skills on low risk hobby projects.

I find I get far more value from this than trying to solve challenges I virtually never encounter in the 'real world'. But then again, maybe I'm just not the target market...