How bad was it? Here's how my co-op attempt went:
1. Set Klee as my character for co-op.
2. Make sure I'm at full health and warm.
3. Go to the challenge marker, get matched to co-op.
4. Get white loading screen. (ugh)
5. When the game comes back, the challenge is already in progress, Klee has 0HP, there's a revive prompt.
6. Revived Klee is on the other side of a force field unable to help and Diluc is yelling at her to click the light.
7. There was no light.
8. Challenge failed.
tl;dr, co-op has a loading screen that murders my character before the battle even starts. Solo doesn't do that.