1. Bland layout and/or concept.
It's basically Halatali HM(which was also boring imo) but on Gilligan's island. Not only it doesn't really try to hide the fact it's a mere hallway, but it even mostly used the pathways of normal mode. You can still make a hard mode dungeon feel like it's something completely different. Examples of something that tried this was Lost City and Copperbell.
There's also the "show your power to these people!" tidbit except you're fighting the Maelstrom. That's cool and all, but didn't you guys already do this with Halatali?
2. The boss fights aren't engaging.
The first boss(if you even want to call that a boss...) is just straight up adds. Boss fights like these can be fine if executed in an interesting manner, but this dungeon failed to do that. They were just....adds. That's it. Nothing really dangerous outside of a few conal aoes that gave vuln stacks. They're just...adds.
Although I am annoyed that Ymir gives the middle finger to melee, that isn't where my gripes come from. To me, the mechanics only existed there as a "oh, just make the boss do this so the player thinks there's some type of difficulty" rather than "let's make these mechanics interesting like what we did with sohr khai!". Some examples are the shell switching that completely ruins openers and the ice skating rolling rink of death(except pretty much a good chunk of the place is death).
Mistbeard in concept is a cool boss fight. Everything else, not so. The fight is literally a striking dummy since most of the mechanics aren't dangerous. The main worrying mechanic, Dirty Venom isn't even always a threat. There were times where my party didn't even get the debuff at all for the entire fight.
This dungeon singlehandedly made me stop playing Expert. It's already extremely easy to cap lore tomes(seriously, have you tried aquapolis? It FEEDS you an unhealthy amount of lore tomes) as it is, and there are better sources for esoterics. It's just not worth it.