One problem with this is that browsers don't behave the same. For example, iOS Safari prevents multiple pop-up windows to be opened by a single user interaction. Each one requires clicking back to the original page and allowing the popup. Now you might say, "Why would you ever want to do that?" But there are always going to be edge cases—in this case it's an integral part of one of the features of autotempest.com. But that's just one example. And the only way we can detect whether that behaviour is going to be blocked is by checking the UA.

I can understand why this is a good thing for privacy. Like many things to do with security on the web though, it's just a shame that bad actors have to ruin so many things for legitimate uses. (The recent story on Safari local storage being another example of that...)