I run 100W UV and an Ozone reactor in a 145g tank, so way overkill.... My tank is jam packed with pods.
Worth it? I wouldn’t run a tank without it. I don’t quarantine and I added a Porc puffer and immediately noticed it had cloudy eyes and just swam in a corner....... A week later it is eating like a pig, swimming the tank, and looks great with bright blue eyes.
No clear cut answers on this. It's dependent on the size of the UV, It's effectiveness bulb wise and flow. Also the size of the system and number of fish and what type they are.
Most hobby grade UV sterilizers are too small to effectively remove free range protozoan and are used to manage them. These UV units are effective in keeping water clear from bacterial and algae blooms.
Their effect on copepod, amphipod, Isopod populations is usually minimal. Due to the nature of these bugs as they tend to stay on the substrate, rocks and surfaces in the system and not in the water column.
If the intent is to use one because of a parasite or disease out break, it's best to quarantine and treat as needed.