The biggest upside compared to other code challenges I tried (HackerRank, CodinGame) is that the code is ran on your machine, and the pass/fails are from unit tests. So you code using your editors in a real development environment rather than in a web based sandbox.

Also, as the checks are from tests, there are no single right answer. I've seen some code challenge services (can't remember the name) that would mark fail if you didn't write it exactly as they were expecting, to the line break.

For learning a language, I like and have used exercism but personally I prefer Codewars (and am a subscriber) because while exercises alone are a great way to get experience, perhaps the most valuable part of Codewars is being able to see what better coders did to solve the very same challenge.

I have learned so much about what was capable in my favorite from those other answers.

I believe when exercism came out it's primary goal was to help you get ramped up in a new language, however I'm uncertain if that's still the case.