Jebus, it's not a matter of whether or not the stage has hazards or not, but rather how interfering, random, or otherwise threatening the hazards are in the first place.
Halberd, for example, has very powerful hazards, but you get like 5 hours to react to them and get out of the way. The stage is legal because the hazards are hardly a problem there, but rather something the players can work with to turn in their favor.
PTAD has cars that come without warning on some of the phases and kill you at really low percents.
GGs has random bomb blocks raining down from the sky that do 30% and considerable knockback, as well as the exploding apple and wall of fire glitches.
Norfair is borderline, but the reason it's banned is due to the fact that the lava wall can actually kill you at modest percentages, the hazards tend to cause some weird box-in scenarios sometimes, and the lava plumes are very hard to act against and can kill you.
Having hazards isn't what invalidates a stage. If Spear Pillar(Dialga or Palkia, not Cresselia), for example, didn't have circle camping enabled on its stage, one could easily make an argument for its legality because the only hazard, the laser beam, doesn't happen all that often, has some windup, can be shielded, avoided by grabbing the ledge, and SDI'd out of(There's also Dialga jumping onto the stage, but that's given away by a roar with nothing happening immediately afterwards).
In Brinstar's case, the acid rises above the stage a mere 3 times a match, at around the same time every time. You can see it coming from the background, and you're forewarned to its coming by the screen shaking.
It's not that easy since there is no way of always knowing whats going to happen next
Except we do know, because given the facts we know about the stage, the best way to go about handling the acid would be to watch for the screen shake around the times when the acid is going to rise to a Level of 7, 8, or 9, which means you know when the acid rises at that time, you have to move out of the way. From there, all you need to do is watch the background to approximate when the acid will reach your position, and then act appropriately to it, as well as your opponent's movements.
Screwing up here gives you only 15% damage which doesn't even kill, a little bit of hitstun which you can act out of very quickly, and it sometimes puts you at a disadvantageous position, but you can use the heavy DI enabled by the acid to cope with this. What's important to remember, though, is that the match is only interrupted by this hazard only THREE times a match. We've seen matches on other stages, such as Delfino and Pokemon Stadium interrupted due to some of the transitional phases for some time now, so why is it suddenly so taboo on Brinstar when the interruption factor is not even as severe?