Why would banking be a problem for passengers on the Flying-V aircraft?
There are two possible sources of discomfort for passengers sitting farther away from the aircraft's longitudinal axis during a roll manoeuvre:


At the start and end of the roll manoeuvre, the angular acceleration will be felt as a stronger vertical acceleration, as you pointed out (vertical as in perpendicular to the cabin's floor).
Between the start and the end, the aircraft will be rolling at constant angular speed (or roll rate), so the angular acceleration will be zero, but the passengers will still be moving along a circular trajectory (around the aircraft's longitudinal axis) and therefore will also be subject to a centrifugal force (parallel to the cabin's floor) that grows proportionally with the distance from the longitudinal axis.


The roll manoeuvre itself may only last a few seconds, but if the above effects are strong enough, it could be enough to make some passengers feel properly queasy, especially if it's repeated a few times as during departure and arrival procedures.

As you suggest, rolling more gently and more slowly would limit these effects. However, instrument flight procedures always involve standard rate turns, with turn radii expected to remain within certain limits, depending on the aircraft's speed. Rolling more slowly would increase the aircraft's overall turn radius and could make it harder or impossible to comply with currently published procedures.