What? I've never heard of this problem. Ditch your guild and find a better one lol.
Its probably not the guilds but how mobile the trial fights are. As with Asylum Sanctorium there is a lot of moving required, and I believe the same is with Cloudrest. And unfortunately Stam excels in melee range, so they have to run from here to there and in doing so lose DPS.
Again. If it's not balanced for everything (Stam and Mag) then it's bad design.
Because we have trials groups that exclude stam, plain and simple.
If you only ever want to run with the most optimal builds ever, you are always going to exclude certain builds that are somehow slightly less than optimal, even if those builds could complete the content.
Because we need trial group finder?
Guilds not taking stam for the new trials?
Find a better guild. If they can't stay alive on stamina just as well as magicka, they don't have proper understanding of the mechanics of both those trials or don't have the proper reflexes. You don't need anything else than Vigor in Cloudrest. Asylum does make it harder on you if you don't slot Deadly cloak to go with Vigor, but that's about it.
Enemy moves? Your strongest DoTs are on your bow bar, and you need to refresh those. It makes no difference.