Except that a bar alt hold can be fooled by pressure changes - hi to lo = overread - as you fly towards a depression.
Given a local QNH and a bar alt to rad alt comparison this isn't a problem. The biggest reason that rad alt isn't used is that most crew change aircraft only have one so a failure would not be good. I can assure you that you wouldn't notice the difference on a 225. In fact, the bar alt is nice and smooth with an accuracy that is more than sufficient for our operations.