Not just OPC/UA, but practically all of your native PLC protocols. Non-deterministic. If you need deterministic communications, you need repeating UDP transmissions (or bare Ethernet frames).
@Yfactor, your line of questioning tends to be where engineers start when they realize SCADA packages tend to have trouble with momentary buttons driving booleans. This topic has been beaten to death numerous times on this forum:
http://forum.inductiveautomation.com/search?q=momentary
This one is probably the most thorough (and contentious):