I think it greatly depends on how much stuff you are carrying. The more weight and volume the more the benefits you mention kick in. It often strikes me that some of the setups I see with either seat bags or with bikepacking bags had way more stuff than those setups were really suited for. When I am carrying 9-14# of gear how high or where it is on the bike doesn't seem to matter very much. As the weight gets heavier I start to think maybe panniers make more sense for me. I can't say exactly at what weight that happens. For moderately light touring, I kind of liked front only panniers or front panniers with a bit of gear on the rear rack (at the time I was using a tent rather than my current bivy so it was on top of the rear rack). It can be a pretty light duty rack in that usage.

I figure that a seat bag probably avoids most of those problems when the load is light enough, especially if the load is split between it and a bar roll or handlebar bag.

I am not speaking from direct experience on the carradice style seat bag though, just general experience with lighter loads in general. Maybe someone who has used the bags in the way I suggest can pipe up and either confirm or shoot down my impression.