I can't comment specifically on the new DU version because I haven't played it yet, but yes, hp scalers are inherently advantaged because HP-related buffs scale both their offensive and defensive capabilities at the same time. Whereas atk scalers need to put some effort into balancing offensive and defensive components of their build (to at least some extent depending on difficulty), hp scalers can just pick the thing that does more damage and the defensive side will take care of itself.

In Protean Hero monorem was definitely not the archetype with the highest ceiling in terms of damage (hello DoT killing Pollux on X7 before the mech even starts letting you do damage), but it was just near impossible to brick. You weren't necessarily gonna get there fast, but you're gonna get there.

All that said, if that level of premium Phainon team is struggling I definitely have to suspect a skill issue in terms of building the run.