After playing both sides, I don't believe 2vs8 is unbalanced.
From the killer point of view, I've won all my matches where both killers were decent and lost off my matches where the other killer was beginner (tried with a friend who only knew survivor)
The won matches often went down to the last gen but progressively killing survivors was too much even for the best players I've versed.
The lost matches were over in a few minutes with at most three kills.
From the survivor point of view of the survivor, I've won about 2/3 of my matches.
I attribute most of my deaths to my teammates making huge tactical mistakes.
e.g. Not healing a rescue, ignoring a nearby pod to work on a gen, wanting to run "forever" before agreeing to heal … right when the killer is on us, never taking a protection hit, running back to the gen with the killer chasing … you get the idea.
I equally attribute most of my survivals to my teammates doing the right moves at the right time.
I've versed all killers and, IMHO, the most terrifying one is Huntress.
Trapper is pretty dangerous too with these nicely placed traps.
In summary, I believe that competence and organisation is a significant factor in the outcome of the trial for both sides.
For me it means that, even with the apparent chaos, this game mode is has some level of balance.
I need to add that, winning or losing, I had fun on both sides.