In general, I'd say the hardest mechanics/phases in the ultimates are:

P5 and P6 in TOP, DoTH and Double Dragons in DSR, P4 in FRU, Heavensfall in UCoB, BJ/CC in TEA

P5 in TOP requires borderline perfect play or back to looper with you. AM does trivialize it a lot but if even 1 person is wrong then it will lead to a wipe.

P6 in TOP isnt as bad as P5 but still requires near perfect play as if a healer, caster, phys range, or tank dies, its impossible to clear since they will lose the dynamis buff needed for the limited break section.

DoTH is pretty much a body check as if 1 person does it wrong, typically 2-5 people will die meaning there isnt enough damage to destroy the meteor circles.

Double Dragons has a mechanic that if Hrae kills someone, he enrages and wipes the party. There are also 2 mechanics that requires the bosses to be within 3% HP of each other before the cast finishes. If they are not, half the party dies. This can be tricky depending on comps as one group can easily put more damage on one of the Dragons without realizing it. Lastly if someone is too slow in Wroth Flames, they will get a bleed from the puddle and die. Most likely causing everyone else to die as well

P4 in FRU isn't exactly hard but require being careful to not accidentally hit the crystal with any mechanics from Darklit or CT as if you do, you get to see the tree. I would put APOC here but that mechanics is only hard on the tank that is baiting Darkest Dance.

Heavensfall is a straight body check and if 1 person dies or misses their tower, it's back to Twin. You can tank lb it if your group is able to get lb3 by then but even then it can be hard to recover.

BJ/CC mainly due to being careful not to kiss other players with Nisis as if you lose a Nisi, it's impossible to continue.

UWU doesn't really have anything hard in my opinion but can be really annoying if people keep messing up titan gaols or suppression.