It doesn't matter how much experience you have with past estimates if one of the steps in the flow involves a scenario where you just can't estimate it. (Happens a lot when dealing with third parties)
We have third parties building portions of our systems and for some of them I can estimate reasonably well their actual net duration because I've had a lot of experience with them, and they've been fairly consistent.
But there are others where there's a wide range, so I think it boils down to whether you can spot a pattern or not.