Think of rubber guard as an omoplata setup. Thinking of it as a "guard" is silly. If it's there, set it up, but if it's not...don't force it. You shouldn't force any other setup, so why try to force rubber guard? It's FINE, but when people think of it as some sort of go-to position it just doesn't fit properly.
In short, use other techniques, because regardless of how good you get at rubber guard, you won't always be able to set it up.
The other thing I'd say is, yes you can get very good at breaking posture, you just have to practice. But once you're very good at breaking posture, rubber guard is just one of many options and you may find yourself to be less inclined to use it.