I've had pretty good results running a modest vinegar does in my mixed reef tank (RBTAs, leathers, zoas/palys, misc montipora, no acros, no LPS). I run an automated system and a couple of times I have accidentially let the vinegar run out - nothing bad happens quickly, I'll notice little bits of nusiance algae starting to pop up here and there which reminds me to check the vinegar. I replace the vinegar jug and in a week or so everything seems back to normal. I suspect that if I was dosing more aggressively then there might be problems (most likely depleted O2 as the bacterial population dies back) with the dose cutting out, but at least at a modest dose my nutrients stay lower than without and I haven't seen any problems other than some cyano growth that may be related to the carbon dosing.
In terms of fish health, as far as I know the only concern is depleted O2 if the dose cuts out or if you have a power outage.