Reactive evolution is almost like the defensive equivalent to reality warping, where it’s intentionally broad so tons of stuff can fit into it (in this case pretty much any defensive growth).
The only thing that should be a universal limitation is the requirement to be threatened or effected by something before reactive evolution activates.
We generally apply the limitation that characters can’t adapt to having reality warping abilities applied to them without having feats of doing so already.
Other than that, there are usually limitations you can assume based on how it works, like with most powers.
For instance, with reactive evolution based on physiology changes, you’d expect an inherent limitation to how those changes interact. In this situation, it likely would not be possible to adapt to two opposite stimuli at once, since the different changes might require contradictory changes to the body. So for instance, say there are three characters, A has reactive evolution that goes through physiology changes, B has hax that activates when A looks away from it, C has hax that activates when A looks at it. If A adapts to B by growing clear eyelids and gaining 360 degree vision so it can always view its surroundings and see B, that might conflict with potential adaptations to C that would require limiting its vision.