You need water in an emergency and your only option is an non potable source therefore I have a UV light bottle for just such an emergency. If this is your use case I'd say get life straws. They'll work better and you don't have to worry about toxicity from bacterial death. Wherever you're going that you have the option of carrying a UV light source for water treatment you'll have better options you can carry to treat water.
In my opinion these bottles are sold to be trendy and have that's shown by the marketing. You'd never want to rely on this for treating non potable water. It seems more about people too lazy to wash their reusable water bottle every few days and thus killing off the bacteria that starts to grow inside that gets transferred from your mouth. Gross thought, but just wash the bottle.
UV add-ins for water makers are a peace-of-mind option for boats with very large tanks that have opportunity for fouling; they are by no means common in the water-maker loop except in the very high-end; the high pressure membrane filters do the job just fine.
plus side : most sailing offers all the UV you could ask for. Grab a bucket and hit the deck, no need for future-bottles.