What could be detrimental from doing reef aquarium water changes??

Water changes are the easiest way to go and natural is usually better, as in cheaper.
One thing though, when you ad water that is not perfectly matched to the pre existing tank water, you get a PH reaction, not a good reaction, usually it will make the PH unstable for a little while, or in some cases the PH may stay low for a time.
If your tank is teetering on semi collapsing due to stress like many are, you won’t see it in testing, it will manifest in stress related damage or more usually a parasite explosion may occur.
This means that if the tanks life is already in stress and one or two have a little parasite farm on them teetering on falling prey to that stress, which always affects their immune system, from then they are fodder for parasites.
You can believe what you like, but protist parasites as in velvet and ich are always present while you keep inverts, no matter what you do!!!!

To explain how parasites are encouraged while fish are at stress, fish have two immune responses, one external and one internally; the external is built into their mucus coating and will fail with hours if their immune system is compromised!
Just like us in a way, if we get cold or stressed our immune system becomes weak and the cold we may not have caught at all, or we already have in our bodies waiting for a foot hold, like fish when stressed, there isn’t much to stop them now from multiplying and taking over.
Many folks die from the flue and most do not, same with fish, but with them they all get stressed from being in the same eco system, with us, once sick, we get help and support individually and usually in our own home!