Very good question.
Personally, I would imagine that because it is an effect that is optional to invoke, and because of the wording as you point out, that Hubris does count towards the limit, and that it is stopped by Otho.
I think the reason that similar abilities like Hot-Blooded aren't is that they happen automatically when you do something else, and that is why they say "generate" and "add". Hubris says "earn" and "spend" and doesn't happen automatically, it is something you have to explicitly pay for - you just pay with Vitality instead of Favor. Therefore I think it counts as normal Favor dice and has to stay within spending limits.
But a model cannot be able to both use Hubris and Circus Maximus (to get a limit of 3) at the same time - it would have to be both Helleniki and Ludus Magnus, which is currently not possible.