A regular participant in the sci.lang newsgroup on Usenet compiled a partial list of irregularities in Esperanto a long time ago.[1] Basically, the whole project of building Esperanto as a constructed international auxiliary language was full of internal inconsistencies, especially in word derivation,[2] and full of bizarre arbitrary differences from any natural human language that just make it hard to learn to speak fluently.[3] I have lived in more than one country, and have studied Esperanto quite a while during the 1990s, but it's never had any day-by-day usefulness for any practical purpose for me that any other language has had.
[1] http://www.zompist.com/kitespo.html