There are a lot of ways in which companion behavior could be antithetical to dungeon or trials success, and I'm guessing ZoS hasn't anticipated and properly handled every single one of them. So testing seems called for, in areas such as:
1. How good are companions at avoiding or mitigating damage from boss attacks?
2. How good are companions at avoiding or mitigating damage from environmental attacks?
3. If Drodda stuns a companion in Direfrost Keep, will s/he rapidly break free? (If not, the fight will be very difficult for everybody there.) Or will Drodda never choose a companion as the stun target?
4. Will a companion block the Warden of the Shrine's heavy attacks in City of Ash, reducing the fire left on the ground?
5. How do companions deal with the portal mechanic in the last fight of Imperial City Prison?
6. How will companions deal with the potion mechanic in the last fight of Stone Garden?
7. Can a companion be tagged in the last fight of Ruins of Mazzatun, and what happens if s/he is?
There are many more questions of this kind one could ask, e.g. for any dungeon or trial where positioning matters.