I think in theory, level 90 dungeons are more difficult. I mean you have stepping in AoEs giving you Doom in a lot of 90 dungeons. You also have it not telegraphing a lot of things with orange aoes and instead expecting us to just figure it out based on where the boss is looking or environmental clues. There is also the boss that shoots lasers that involves observing the environment to identify the safe spot. They also aren't afraid to make us do a little bit of maths, like observing the order water spurts will come out in Dohn Mheg or the order the floor will explode in the second boss of Troia. It's just that since we farm this level 90 dungeon for 8 months to a year, it quickly becomes "easy" and gets boring to keep doing it to cap tomestones. To the point hunts are genuinely more fun.

So part of the solution would just be merging Expert Roulette with Level 90 Roulette to provide more variety.

The other issue is Yoshi-P said the 2 sidequest expert dungeons will be more challenging. He's not afraid to make the sidequest ones more intense. The problem with that is we only get 2 in the entire expansion. They discontinued sidequest dungeons. We could say "well they make variant dungeons". But these are not something you get in expert roulette or farmed for as great a duration of time. Which is perhaps part of the problem of separating this dungeon system from regular dungeons. Variant dungeons are also flat out designed to be soloable and to ignore the need for the trinity.