I today ran into a problem with omudpspoof (rsyslog's UDP spoofing output module) that only occured when multiple output threads were used. If that perquisite was given and a larger traffic volume was generated, rsyslog relatively quickly aborted - often even during startup, otherwise very quickly thereafter. Diging down to the potential troublespot, it turned out that libnet is not threadsafe (and I have to admit they do not explicitely claim that). That was a very quick hint, and I now protect libnet calls via a mutex, ensuring that only one thread at a time will ever call into libnet. Of course, this somewhat limits concurrency, but for obvious reasons stability is far more important. There was also recently some discussion on different-kind problems with omudpspoof. While I do not think that this was releated to multithreading (data was malformed, what I could not reproduce), I can not totally outrule that. Will try to follow up with that on the mailing list as well.
The problem probably took quite some while to surface, as people usually have a single UDP spoofing action AND do not run it on multiple threads (given the i/o-boundness of this action, you probably can not gain much be using more than the standard single instance thread).
If you are interested in patching older versions, the patch is available via git (use the raw link to obtain a file usuable with patch). New releases are due soon.