True, but except for cheesy Nightfall Phogoth, all the other bosses at least let you enter the room to fight the boss in.
Under no circumstance do you want to really be in Omnigul's room. You will very likely die. Imagine fighting Valus from the adjacent room? And then give him an elemental shield you have to drop everytime you pop back around the corner.
Yea Omnigul is the only strike (and I don't mean jut nightfalls) where you don't actually fight while staying in the same room as the boss, it's hard even while playing ROC as a level 32. It's one of the worst encounter design in the game because of what you said ie. agile boss, respawning enemies along with the location of the boss in the room + the shield and wizards all of who have a never ending booming attacks.
It's the entire reason I always skip that strike if it shows up in ROC, I also skip Undying mind all the time with no regrets because it's one of the least imaginative strikes encounter design wise again. Just the last story mission in reverse with wave after waves of enemies who don't drop shit. Devil's Lair, Dust Palace, Archon Priest and even Valus are very enjoyable strikes.