The feeds that this class generate do not follow W3C standards for syndication. I spent hours modifying my code over and over to try to get it to spit out correct RSS2, RSS1 and ATOM feeds from the same set of data. Eventually, I came up with a working script, but I had to re-write a lot of this class in order to not have it put in tags that either didn't belong or were named incorrectly.
Also, there was no way to just get it to return the XML contents of the feed, rather than just going ahead and printing it. This was important to me because that's how my whole site was designed: nothing prints until everything is constructed.
I have written a patch that provides the things I have discussed. Still, this class needs more work, and instead of just calling generateFeed(), one must call 'header("Content-type: text/xml");print($feed->generateFeed());', so this isn't exactly a drop-in replacement for the original.
The link to my patch: http://sites.google.com/site/danchurchinc/software-patches/universal_feed_generator-correctness.patch?attredirects=0&d=1