Megataur was just an example. There was a time when he was considered very powerful, and he peaked at P+2. I still used him for a long time after that, because while everyone else promoted him, I kept him at P+2. This made my Mega a lot tougher than his P+3, P+4 and even some Orange buddies.
He was also pretty good because, for a long time, he was the only tank who dealt a significant amount of damage.
Even now, he's not a junk hero. He's a little above average as tanks go. I often use him in war, because he's fast enough to protect my ninja from crabs, and this is against the top guilds on server 4. And when the tank buff is on, he can do a ton of damage. If it weren't for spectral and bone, he could plow through Expedition on his own.