With all the talk about how important it is to sanitize your equipment when brewing, is it acceptable to use a UV sanitizer light on a finished product just to make sure there is no harmful bacteria left in the bottle/brew?
I don't think it's a good idea, but might depend on product. You know why brown bottles are more popular than green or clear? Because light creates bad flavor and aroma in most beers. In my country it's known as skunks aroma. Strong UV lamp will do the same, only much faster. As far as I know, wine doesn't like light either. But I believe there might be some brew that would survive without harm. I just can't recall any.
Sun gives about 0.01 to 1 watt of UV energy per square meter*. UV lamps give between 8 and 20 watt / square meter. Some possibly even more. It only takes two hours to distinctly skunk your beer under direct sunlight. So skunking due to lamp use will happen even faster. This relation is not linear, so it would be up to you to test how much bad flavors lamp would introduce, but I feel convinced amounts will be significant.
There is also another problem. You would either need to pump your beet thorough special installation, risking contamination at later bottling and thus preventing any benefit, or you would try to pump UVB and UVC thorough bottle glass. Glass tends to be opaque to UVB, and probably to UVC*, and it's UVB and UVC that's used for sterilization. So what you would probably sterilize that way is outside of your bottles only. And that's even when we didn't count bottle manufacturers' effort to mitigate skunking. By the way, skunking is also caused by UVA, that's why even brown bottles does not prevent it completely. Sadly, UVA hardly has anything to do with mass killing of bacteria. Even more sadly, most UV lamps gives you some from all three kinds, so ineffective sterilization may still be effective skunk.
* I'll be grateful for hard reference on these points.
Nope UV is bad for beer. It will make it taste horrible (skunky is one of the aromas that will be produced). I you mean harmful to people, then don't worry. There are no harmful bacteria in beer, they can't survive in the beer and bottle environment. If you mean harmful to the beer, then filtering and pasteurization can remove or kill these organizims. Many beers, usually ones high in alcohol, however, are aged on lees or are otherwise bottle-conditioned and are not pasteurized or filtered.