I’m getting ready to build a house soon myself and I’ve looked into them as well. My family has always been involved with home building so do have some knowledge on that front. From what I’ve seen they don’t seem all they are cracked up to be. Lower/harder resell, may not be permitted in some areas, may require engineered plans, etc. It looks like some places are becoming more friendly to them but require traditional framing on the exterior walls between post which will probably eat up any savings.
The more I looked at barndominiums it became really apparent where the cost savings came in, your building a cheaper, less quality building overall. If you finish one like a normal house I doubt the finished sq/ft price will change much. I agree with Def90 they seem to have popped up as a way to skirt regs/taxes.
The only advantage I really see to post-frame builds is if you are building it yourself you can spread out your time frame for getting stuff done. The roof goes up fast and everything can be built under and around it later.