I started it once when I was super under-utilized in my day job and was looking at ways to stay stimulated.

I figured I'd start out getting approved as being able to do useful work in a particular CMS in a particular language (Where I expected the acceptance criteria to be low (and the size/complexity of engagements to be low) -- customize a listing of content, build a custom form, customize how content is themed, etc...

After typical screener quiz etc. and a Skype interview, there was a take home challenge that resembled "build a minimalist custom ERP with <such and such acceptance criteria> and <such and such custom business logic> shoehorned inside of WordPress" , probably a good 20 hours of work minimum if you were using an MVC framework with decent developer experience, let alone trying to bang it together inside a CMS without much support infrastructure for such things.

I declined to complete the challenge.

I might have had better results going straight for a language + framework actually suited to creating custom apps, but it didn't give me a great impression.