I don't do PvE missions often, but the few that I've done in the ESTF genre. And this is more or less for the Borg: crap...forget the name...it's the one where you have to protect that stupid derelict Klingon! (shows how much I've been playing) Did that mission get more difficult all of a sudden, or is just me? And if the mission is failed...i.e. every borg ship materializes in the zone all of sudden and wipes out all 5 players, resulting in "Mission Failure"; why cannot we leave the board w/o getting the 1hr barred game penalty?
This is currently a bug, the mission is Cure Space (Normal and Elite). There has been a significant amount of discussion about this problem over the last few weeks. Hopefully, Cryptic will be able to fix the problem in the future.
If you don't want to get that damn punishment for leaving an STF early. I found that turning off my internet then waiting to get logged out by the server useful. But you have to wait up to 15 Minutes to log back in to the game. Hope this helps.