I'd say my only issue with Challenge mode is it'd be nice to be able to invite partner characters for those who aren't comfortable playing solo and don't know anyone who plays.
As a final note, I'm tried CM a few times and am in NA, but due to health-related reasons, my sleeping schedule is all over the place and I'm not always playing at a "popular" time so I have only ended up playing it at least once.
I'd like to try it again, but definitely with more people at some point.
I do think more ways to get CM would be nice at least. People can say this is an optional mode, but someone may genuinely really like the weapon or are working on completing the compendium and it's not really as optional in that case ^^;