I think they use "balanced health" when talking about long-term effects, about one´s lifestyle, exercise (or the lack of it), etc.
They would, more likely, to state that "your health will have side effect" (or you will live with side/adverse/negative effects"). Or that it will worsen your health, or you could say that one´s health never will be normal again (it can get episodes of getting better, though).
As for the sentence "with (or under) a terminal illness, our health is never balanced", I won´t say it´s wrong to say. What I would point out is that it´s not common (and perhaps not much natural). And if we would say that we mean lifestyle (hence a disruption of long-term effects), it would be obvious. Hence, unnecessary. It´s my thinking, though.