I hope you like grey answers with lots of caveats
Take each opinion here with a grain of salt - even if it sounds right. I'm pretty new as well, but when I was doing the research and shopping around, these are the conclusions I came to...
First, it's definitely one of those issues where you have AMAZINGLY successful tanks both WITH and WITHOUT UV.
As far as destroying good bacteria - YES it absolutely can an does destroy good bacteria. Anybody who says differently doesn't have a real understanding of what UV actually does. UV is indiscriminate. SIZE of the lifeform and DOSE of UV is what determines what gets killed and what doesn't. The higher the cumulative or one-time dose, the larger the lifeform it kills. A higher dose can be achieved by some combination of longer dwell time, higher wattage UV, and frequency of exposure. This is why there's so much debate about "what dose should I be running." But in general, if it kills LARGE lifeforms (i.e. fish parasites) it will BY THE LAWS OF NATURE also kill anything smaller.
Here's Aqua UV's dose chart with my own tank's info highlighted as an example:
Notice how a SLOWER flow rate means a HIGHER dose. Slower flow means longer dwell time. That's why they recommend a slower flow rate for LARGER organisms. But that means it will also kill anything smaller than that. That's also why they don't recommend those higher dosages for reef tanks (see below).
Now consider what that means for choosing a size... As an extreme example, if you get a 240 watt unit for a 70 gallon tank you'll be NUKING EVERYTHING that passes through - good, bad, neutral - no matter HOW the flow rate is set. Oversizing your UV just means it will be easier to kill things, which may not be a good thing if you'll trying to selectively kill SMALLER things (bacteria, algae, etc.)
From the Aqua UV user manuals:
Now if you dig into Pentair's recommendations, they're quite a bit different. I don't remember them specifically but when I was shopping around that was one of my frustrations. This is one of many examples in the industry of a subject with not enough research behind it. You have the 2 biggest players in the UV game making different recommendations, BUT people are having success with both
This is what AquaBiomics has to say about UV sterilizers:
DNA testing for aquariums and aquaculture
aquabiomics.com
Given that Aquabiomics is not trying to sell UV sterilizers, I'd take their input pretty seriously
Even so, there are just SO MANY VARIABLES to consider. Some way one person runs UV will work for them, but not for somebody else because of some other variable (or more likely many variables) in their system. And there are of course TONS of people who DON'T run it and have wildly successful tanks.
Welcome to reefing 