Well, it does not seems to use a cache feature, which can become pretty harassing for the server, in case you have many users.
And the feeds URL are outrageous, I don't think they would be usable with any rss parser, and, mod rewrite would be a bit complex it seems to shorten them.
http://www.phpbb.com/phpBB/viewtopic.php?t=254606 this one does uses cache, and if there is not en user friendly interface, you can still provide links to feeds on every forum.
Again, it's outputting posts URLs, so it's not that good, as far as Search Engine Optimizing.
Then it's up to everyone to choose what to do.
I'll work on an SEO RSS solution for phpBB quite soon
