I think a lot of the frustration right now (and take a look at reddit if you think this is minor) is that at one point it was beatable by hitting auto 100 times and waiting for Mace to forget the "reinforce" button. However while that bug appears to have been fixed, the fight has not been re-tuned to make it possible.
It also makes me wonder if CG designed it to be best with Republic ships so we would be forced to farm DS for arena, Rebel for Thrawn and Republic for the ACKBAR mission. Because an offensive rebel lineup struggles against the 100's of thousands of EHP over the ships.
Also, there does seem to be a reinforcement issue where the reinforcement cooldown seems low. Unless it is due to the valour.