In Stunner, in both PAM version 7.1.0.GA as well as latest community, I can't attach boundary events to service tasks. E.g. I can't attach a Boundary Catching Error Event to a WebService WorkItem, or any other custom WorkItem for that matter. It does work on other nodes like script tasks and human tasks.
This basically means that you can't do any proper error handling etc. when working with Service Tasks, which are basically the tasks we use for calling to external systems. Support for proper error handling is critical on those kind of nodes. That's the reason I raised this as a blocker for 7.2, as it prevents us from modelling any real-world processes.