D&D 5E As a DM, what monster abilities do you find difficult to run?

[Ruin Explorer]

I find this to be pretty common, but I also find the CRs in the MM to be fairly accurate (with some exceptions) when you account for all the features and traits. Others do not agree.

It's a lot less bad than 3.XE, that's for sure. 5E's CRs are mediocre rather than an active hindrance. Some are highly accurate, some are dubious, but where they're dubious, it's more often in the safer way of overestimating a monster (a least a few seem predicated on an unwise assumption that a monster will last like 5+ rounds), rather than the other way around.