However, yes they require a bit more balance/treading at the surface to remain upright. It's not a "training" issue or a myth it is pure fact...
That may be YOUR experience, but it is NOT a universal fact.
I have never needed "more balance/treading at the surface to remain upright" while using a BP/W on nearly 500 dives. Neither have any of the dozens and dozens of people that I dive with on a regular basis. Neither have any of the scores of people with whom I've conducted DSD's with while they were wearing BP/Ws.
In fact, the only time I HAVE seen this issue is when people overinflate their wings while at the surface, especially if they have a much larger wing than needed. It is a either a "skill" issue or a "training" issue or perhaps a "configuration" issue, but that does not make it a universal fact for everyone else who wears a BP/W.