Any endgame system based on progression and actually playing well is always going to turn into a fucking shit show when you partner up with what are basically randoms. And that's what the group finder does.
It doesn't matter if you're finding groups in General chat, through the group finder, through a website, or a completely random group maker. It is going to create tension and anger as your 35 minute run ends in failure.
Personally, I think the penalty for failure is too harsh and the fact that one diva leaving the group in a strop instantly wastes the key and makes sure nobody else gets anything either is just crap. Keys should only go down if you make a conscious choice to do that. Go up when you beat the timer. Allow replacements (and invalidate the timer) if somebody leaves.
I only run in a guild group. And frankly I don't even do that most of the time. The gogogo mentality was already a cancer before M+, and basing an entire game mode around it has only made it malignant.