I run one tank with UV and one tank without:
Water clarity..........both tanks are just as clear.
Algae control, bearing in mind both tanks have readings for the main 4 you would test for........I get a couple of very small patches of GHA now and again in the tank without the UV and none in the tank with UV. I get the obligatory film on the glass quicker and thicker in the tank running UV though! There is Chaeto and Calpura growing in the sump of the tank with UV but not the other tank, so it could be that which keeps GHA out of the tank and not the UV.
Parasite control............it only acts on what passes through it.
Heating up the water..........I haven't a clue about this. It isn't something I can see would happen with my unit because if I turn it off and take the bulb out right away, it is cold. Even if UV did warm the water then surely it means your heater doesn't have to work as hard. That would be a positive point.
Pellet reactor............I haven't seen it on the instructions but came across some info during the week about running UV with a pellet reactor. This has a negative effect on nitrate reduction, according to D-D (the aquarium solution). It slows the process down.