Pulled this off the old googleshttps://www.dandwiki.com/wiki/Myconid_(5e_Race)
I can see the slowness being a potential burden to the group. I can see the aversion to sunlight being a pain in the ass, and potentially the telepathic communication.
That's just my best guesses on the matter. I can see a DM being hesitant, at least. This is something that would need some careful thought.