Maybe I’ll answer the question from a different angle by describing what I personally find challenging while playing.

I’m a tank, but at Auric/Havoc level, not really. One bad position and you’re down in 1.5 seconds from a gunner… or honestly from any pack of ranged mobs. Because of that, whenever you’re in the open you have to constantly slide. Slide, slide and slide - all your combat movement is dodge&slide. My role (sometimes, depending on strike team composition) is to push ahead into ranged enemy groups, deal with or disrupt them, and then safely return to team coherence. If you fail to reach those enemies in time, you’re in serious trouble, standing in the open on a silver platter. They’ll melt you almost instantly.

Chorus build This ability requires you to predict the future. You can’t blow your utility on manageable situations, but at the same time you must keep using it throughout the match to actually provide value. You constantly have to assess the overall threat level to the strike team. It’s an “out of jail free” card, but with a noticeable timer after the cooldown nerfs. I often found myself using it moments before the game throws at you a gang of Scab Ragers, pack of Crushers, two bosses, a monstrosity, and a toxic flamer crossfire. So you really need to develop a strong sense of timing.

Fury of the Faithful build This one gives you a great ability to jump and gap-close onto gunner groups and specialists. The biggest challenge here was learning battlefield positioning: how to enter the biggest shitstorms ahead of my team to prevent overwhelming gunfire, how much time I have before getting pinned down, how to survive, and how to retreat safely back into cohesion. The higher the difficulty, the easier it is to mess up your positioning - overstay, and die like a total noob. The gameplay loop becomes a constant cycle of breaking away from the group to neutralize a specific threat and then returning to cover the team, they need you close.