D&D 5E Is stoneskin underpowered?

[MostlyHarmless42]

Dominate person. I bet most parties hate when the bad guys use that on their barbarian...

See you say that, but in my experience Dominate person tends to be lack luster due to it's "save or wasted action" aspect, particularly when doing an ongoing aoe effect or something that affects multiple party members is more reliable. Your milage may vary and it helps if a monster has legendary actions or can still attack while casting a spell, but I often find a multitude of reasons for spellcasting Monsters to just use their concentration for other spells.