Most of them are just plain slow, borderline helpless. Like all fights, having more people makes things quicker because everyone who hasn't got aggro can pour all their stamina into damage, but solo is still pretty easy. Solo is just a slow burn, that doesn't afford too many mistakes, but mistakes are really hard to make.

I'd rather take on something that is way more dangerous, but also doesn't take 5+ minutes to finally kill. And if it does take 5+ minutes it better be because they have more than one phase. In terms of behaviour Eikthyr probably would have been one of the better ones if not for its duty as a weak tutorial boss.

I would give the devs big props if they made an 'elite' endgame variant of every boss. But then again, they would probably design them to take ten years to kill.