Wrote an article a few days ago on the BPF new architecture and effects that it might have.
I spoke with Roger Gilchrist, a Principal Program Manager with the Product team during the eXtreme365 conference and he mentioned that using the "Process Stage" fields are actually not supported if you read the SDK very closely. This document is a very long document and it is not very clear in the UI and there is no other very obvious way of solving triggering on stage changes than the triggering on Process Stage changes, with which he did agree.
The important lesson from that, I think is that not everything you see in the UI is actually supported for use.
MVP, Founder and CTO at CRM-konsulterna AB