200 Mentor Roulettes later...
-Healer duties were ~90% dungeons (quite a few Qarn Hard, don't recall getting Sastasha Hard/either Pharos/either Gubal/Dusk Vigil/Vault/Fractal Hard/Swallow's Compass/Holminster/Dohn Mheg/Zot/Stigma). Next were EX trials (a few Shiva, a few Great Hunt, a couple Garuda, one Thordan that got noped out of), guildhests, and backfilled alliance raids (lots of Nier). Just a couple normal trials, and can't recall any normal raids or fresh alliance raids. Only particularly long dungeon was a 40-minute Grand Cosmos.
-DPS duties (did maybe ~20) were generally guildhests, EX trials, and normal raids. Only a couple dungeons, including a 40-minute Dead Ends.
-First and only tank duty was another Great Hunt EX where the healer (new and doing it for WT) struggled to get behind the bison and couldn't handle curing the burn DoTs. At the very least, they learned it could be done unsynced instead of queuing through DF.
Although queuing as a healer means you have to move at the tank's pace in a dungeon, you still have more control. Going as a tank puts you at the mercy of the healer, and going as a DPS means the tank and/or healer can struggle. Case in point: power-healing through the Grand Cosmos run resulted in zero wipes, while Dead Ends had five (all prior to Peacekeeper).
It's already been established that EX trials have issues (mentors not knowing which duty to prepare for, mentors being outnumbered or even alone entirely, tanks and healers often being new), but the fact that it takes way too long for replacements to join just further encourages others to leave and requeue. One notable example was getting Great Hunt as a DPS, watching the healer instantly bail, waiting several minutes, rejoining the same instance, trying to switch to healer, and getting put in a dungeon instead.
You're not guaranteed to have sprouts/returners or first-timers, but there tends to be at least one of either. The latter are great for second chance points, though not having the former won't even assure a smooth run (e.g. Dead Ends). If anything, it felt like Trial Roulettes had more first-timers, and that's obviously faster as well.