But all of that pales in comparison to the biggest argument against end bosses: they're nowhere near as exciting as that final battle in HL2: E2. Or the final stages of Left 4 Dead, where players must defend an area against huge waves of zombies until they can escape. Large groups of normal enemies are far better for end battles than a single unique enemy.
Throughout the course of Half-Life 2 and its episodes, players get accustomed to how powerful and dangerous striders and hunters are. At the end of Episode 2, players know exactly how fucked they are. The same goes for the ends of Left 4 Dead campaigns. With any end boss battle, there's simply nothing to compare the end boss to. It's the difference between "Yay I just killed some lizard thing by exploiting a weakness" and "Holy shit I just survived being attacked by ten tanks in a row."
When done well, end bosses can be good. When done well, large-scale end battles can be great.
LOOK WHO CAME: