What’s Wrong with OPML. Three years ago: Technical Problems with OPML. There are other posts too. Like OPML - the XML format with no friends.
It is something similar to RSS. A lot of people use it, but it is broken by design. It is no coincidence that RSS and OPML share the same author. Also the design goals of the language are almost equivalent to those of XML, except that you can not express hierarchically marked up items in OPML as information is stored in attributes.