I ran into one of the main problems with open source. I was using the ultra-powerful tagger mp3bookhelper, and the author stopped developing it. That wouldn't be so bad - after all, the current version is excellent.
But then another common open-source problem piled on. The freedb.org developers left the project. Luckily, there was someone there to take over. But the new owners changed the format of the server's responses to album queries, and now mp3bookhelper doesn't understand the data from there. This cuts its usefulness by an order of magnitude, and I haven't liked the other taggers I've seen.
In other words, the problems are: a lack of stability in new version releases and in the protocols/behavior of the software.