I think the marine melee did the most damage to AvP 3. How many times Rich, Prime, Sweeper and I pistol-whipped an alien to death in a corner gangbang, I don't recall, but it was a disturbingly high number.
I enjoyed the campaign a bit more when I completely ignored that stupid melee. Even the block is wank, and should've knocked you down at least.
Melee between the Alien and Predator? That might be a little more balanced, and could use some work, but the marine just didn't work well at all.
Developers try to make the marine and Alien on even footing phsyically, when that's just stupid. It should always come down to range and surprise during engagements, like every decent Aliens game has tried to do. Just like the films, if you can catch them at range or in the open, you'll most likely kill them, but up close, well, game over.