What eve says make perfect sense to me and it's how I think it is.
The big problem with progesterone is there is an acute lack of research around it, certainly from a trans perspective.
Then taking into account all the variables - we are all different, on different NBE regimes, and the different variations of prog (pharma, USP, wild yam, etc), it is no surprise to hear different theories.
Like lots of things in life, its do the research, a risk/reward/cost analysis, and then make your descision from there.