Wirecast 5.0.3 is working for some people on Yosemite.
Some people are experiencing "button lag" where button either have to be hit twice or there's a response time of many seconds for the function to execute. This experience may vary each time you open Wirecast or happen over time while using it.
Some have been helped by using Wirecast Help>Send Support Information and using the Reset Preferences button.
Since one doesn't know for sure which experience you'll get it might be safe to hold off on upgrading to Yosemite.
Telestream has confirmed they are investigating this.

In generally, I'd hold off upgrading to a new OS until a developer releases an OS specific update or officially states they've confirmed one may not be necessary. For example, Apple hasn't released any OS updates for their ProApps (FCPX, Motion, Compressor, Logic) and there are reported issues in many of those apps.