> have you done migration between the two? if so, how hard was it?

I tried the auto-migration in OPNsense (backup from PFsense, restore to OPNsense) a couple times. Both times it got it mostly right, but whatever it got wrong blocked pretty much all traffic and was difficult to figure out why because everything looked right. I gave up and stayed with PFsense, but figured if I ever really did want to switch I would start from scratch.