What Snow says.
Holminster was always notorious and filled up quite a few pages of the TftDF thread with some delicious horror stories.
A bit like Vault and Bardam's.
It has some spicy pulls when going wall to wall and the bosses have decently high party damage at a good rhythm so all in all everyone has to do a bit more than Netflix afk or push random buttons to make it a smooth run.
It's also one of the few dungeons where you really notice that DPS are not nearly as useless as people like to say. A pull dragging on for too long has a good chance to end with a wipe.

The difficulty wouldn't be noticeable if most other dungeons weren't significantly easier. It's just the spike that catches netflixers off guard.