it is undisputated that peroxynitite is BAD because it is an oxidizer.
Nope. Peroxynitrite is not completely evil 100% of the time, and peroxynitrite scavengers are not completely good 100% of the time. Our bodies to produce scavengers, but evolution hasn't provided us with a superabundance of those, since there's an optimum level for health. ONOO- plays important roles in our bodies, at the appropriate levels. One task is to break down IDO, at the appropriate rate. We need IDO, and we need more during infections or other cellular hazards, but we need it to go away afterwards, rather than catalyzing TRP with no limit. Thus ONOO- is a mechanism for maintaining proper levels of IDO. ONOO- is also critical for various types of signalling in and between cells. So, for a healthy body, artificially boosting ONOO- scavenging may not be a good thing. For an unhealthy body, the optimum amount of ONOO- and scavengers depends on the situation.
We don't have a complete understanding of how all the various mechanisms in the body work together, so we can't be sure that a treatment that looks good from one theory won't cause harm via one of its other effects. So, if you take a ONOO- scavenger and it makes you feel better overall, you might keep taking it but should remain vigilant for unwanted side effects. If it doesn't provide a noticeable improvement, I think it would be unwise to continue taking it on the assumption that 'scavengers are 100% good for you'.