Took Seraphim at level 20. The button stayed grayed out the entire time. I was unable to activate even once (and that game went to level 26.) I died a couple times after taking it, even, and it never once became usable.
EDIT: Figured out how to reproduce it. If you’re channeling an ability when you select the bug, it goes on your hot bar grayed out, and never ungrays, and you can never activate it.