I don't know why I have to keep explaining this. Brisntar's acid does rise up with some variance on what time it will choose to rise. With that said, if we had no other external cues, there'd be no way to tell when the acid is going to come up, and we'd have a reason to ban the stage on randomness.
The thing is, we have TWO other cues on rising acid. The screen shakes first, and THEN the acid rises, at which point you can very clearly see it coming up from the background. So... even if there is some variance, you just have to:
1. Watch the timer for when it reaches times AROUND the times when the acid usually rises above the stage.
2. At that point, you pay attention to the screen, and wait for it to start shaking. At this point, you have been given the cue that the next time the acid rises(which will be SOON), it's going rise above the stage.
3. From there, you just have to watch the background for the rising acid, and act appropriately.
Slightly random, but very easily handled by both players, beyond a shadow of a doubt.
It's just like the cases on, say Halberd where players get plenty of time to react to the weaponry, or Delfino, where players just need to be prepared to handle every stage transition the stage will randomly decide to pick.