There is no "real answer" to this question because there are many variables to take into account.
The impact of a UV unit on your tank depends on how the unit is set up, how fast water is moving through it, etc.
The use of UV units does not directly affect corals, inverts, etc... at least not the ones that we buy from the store and put into our tanks.
If the UV sterilizer has a slow flow rate through it, it will kill (or prevent from reproducing) any small organisms that pass through. This can affect the population of tiny organisms in the water that filter feeding corals use. However, in most cases, this is not going to have a big impact, and can be easily worked around by feeding to compensate.
The benefit of UV sterilizers on saltwater tanks versus their expense and operational costs is questionable in general. Good husbandry practices, a solid QT regimen, and routine tank maintenance can pretty much negate their use. I used to run UV on all my tanks. Now I do not run them at all, and have never looked back.
