That's not really true though, even with substat capping, for a lot of reasons. Firstly because the BIS lists that used relic were already hitting very precise substat tiers, and the 600 gear is going to get maybe very tiny gains, since substats already are very small boosts compared to main stats (assuming you don't drop a tier here or there by losing relic precision), but also because gear isn't where the difficulty of ultimates even lies. It's mechanics execution and consistency

Take TEA. You either do limit cut correctly, or you kill 2-4 people and you don't survive the transition to Brute Justice and Cruise Chaser (you definitely won't have enough DPS to beat enrage with three people with rez sickness). You get into BJ/CC and you handle all the positional mechanics while carefully coordinating movement to deal with all the Nisi nonsense or people die and you wipe. Two people with different Nisis touch while resolving a mech? They die, you wipe. Anybody with Nisi dies? There's no salvaging that no matter how much gear you have: the game just tells you "try again".

The whole fight is like that: recovery options are vanishingly slim and often next to if not outright impossible, and one person making one mistake kills your pull - go back to start, enjoy the next seven minutes getting back to the mechanic you need to practice halfway through the fight, better luck next time

You must execute perfectly, consistently, or you don't make progress. And gear doesn't change that