Her category 'lost in transit' is perhaps the biggest cause of UDP drops. No matter how big your buffers on the send/receive side, if an intermediary carrier decides UDP is not important or a D/DoS threat to their network, bye bye packet... or in some cases of rate-limiting, see you in a while perhaps.
A few rabbit-holes to dive down:
https://www.us-cert.gov/ncas/alerts/TA14-017A
http://www.christian-rossow.de/articles/Amplification_DDoS.p...
https://tools.ietf.org/id/draft-byrne-opsec-udp-advisory-00....