The "conveniences" part you mention when comparing anything against a one piece webbing is not so much about donn & doff. It's about placing different 'D' ring styles (straight/45 degree/fixed 180/etc to get 'exactly' what you want instead of what's given to you. For me, having the correct 'D' ring styles in different spots is a safety and speed issue. I can't be 'no-look' searching for a folded or stuck D ring on my rear hip. Also, pockets / weights / canisters can be placed how "you want them" not how they are given to you. If you hold 'something' regularly in your left hand (cam/flag/gun) then it make more sense to have a right hand pocket / "D" ring to reach an item.
Placing of the D rings and such doesn't have to be mutually exclusive with a "comfort" harness, depending on brands. Personally, I'm using the Hollis Elite 2 harness system, but I reconfigured it from how it comes stock to have d rings placed like a standard harness. This obviously doesn't work with some of the pack type harnesses where they're sewed in place.
The harness I'm using also has all the hardware necessary to run a single webbing harness, so in the future, should I want to transition to a single webbing harness, my cost to convert over is roughly $15 for some webbing. Makes it easy enough to be set up with both options instead of an either or condition.