Maybe this problem doesn't exist at level 51, or maybe it doesn't exist for those with a large fleet to help them. I am up to level 33 now and every Fleet Action I have done so far has been overwhelming for the few players that are there to complete it.
I just tried the Fleet Action at the temple with all the Romulans and my team of 5 struggled to get to the step where you have to put shields around the artifacts. The instant we triggered that step by killing 125 Romulans, everything in the instance re-spawned and we were immediately surrounded and killed. After regrouping, we tried to fight our way to a shield generator but the massive amount of enemies and rapid re-spawn rate of them made it nearly impossible. We managed to shield two out of ten objectives before giving up after spending hours in there.
Why you would make content that is not possible to complete with the minimum listed requirements is beyond my understanding. It either needs to scale to the number of players in the instance or be designed so the minimum players can complete it in less than two hours.
The temple takes a while. Make sure you have a medic who isn't going in guns blazing. I haven't done that one in a while but the trick is balance and advancing slowly. Plan out what your team is going to do and be careful to stick together. Try different kits and weapons and see what works the best. I assure you it is possible. I beat the undine one by myself by creating a photonic fleet swarm. Try and be as creative as possible. They are ment to challenge you. Try the crystalline entity. I've never beaten that one.
Yes, I am a new player. I know they are supposed to be challenging, but they took it too far. Respawns were too fast for my team to advance once we got to the third stage of the mission. Besides, it shouldn't take several hours to complete. It took us over an hour just to get to the third stage. We spent another half-hour dying before we gave up.
The mission also doesn't care who joins, so we didn't have a medic from what I could tell. A couple players were useless, too.