Well look at it this way, you have to be in hit-stun for EX Revenge to work. Spells and long range stuff that isn't dirt-obvious as an attack could hit you fast enough that you couldn't get a chance to activate it. I think you still take damage even if it is activated, so that means you could already be in Break state when it goes active, and me knowing how Mu-no-Kyouchi works, the minute you actually "start" a combo, the EX Gauge will only last until the combo ends, then it'll stop.
Basically, if you finish a combo or start to do something with long startup after already hitting them, it's over. That probably means you only have time for one brave combo, or brave-HP combo.
Here's a better way of explaining it...let's say you're getting comboed and activate EX Revenge:
The gauge will slowly drain while time has stopped, this is how long you have to initiate any free hits, so you have to take advantage of it (unless you're too far away, then you're boned, Terra players can abuse that).
Once you land an HP Attack, the mode will end regardless of how much time you had left. Same goes for Brave combos and Brave-HP combos....and like I said before, I don't know if Assists are allowed in this mode, or any benefits gained from EX Mode itself (such as Regen/Haste/Break Damage Limit etc)