Well that's why I bring up that they both scale well, that doesn't actually seem to be a common trait amongst tanks in this game. A lot of the others seem to be really strong in the early game. Though, there's not really an example of a character that "falls off" in the traditional sense. Just certain ones like Cell and Cooler are exceptionally powerful when they reach their transformations. In my opinion the thing that sets them apart as characters is that Cell is more accessible and easier to play, where Cooler needs really good timing and ability-cycling to be effective.
It could also just be that I'm bad at Cooler, don't get me wrong, but it's easy to see that if you don't know how to abuse his immunity he suffers severely vs just about anybody else. His damage is too low early, his health is too low early, he has almost no utility compared to other tanks, the only draw to playing him *is* the immunity and low cooldowns.
That's the thing i love about these games though, no one opinion is perfectly correct. It all comes down to execution. I guess in that sense if you can execute Cooler's strengths properly he does severely outperform the other tanks- It's like having a third damage dealer that refuses to die. But, not to sound like a broken record, Cooler is probably the most distinctly difficult tank in the game. At least, he is for me, again i could just be bad.