Those who drive manual cars are frequently removing a hand from the wheel to adjust an aspect of the car without looking and I would be surprised if the momentary loss of a hand on the wheel would mean manual drivers had more accidents than automatic drivers.
Re: steering wheel buttons in general, it depends on how they're implemented. If you take an old idea [1] and provide tactile locating then it is easy to learn. If you can reuse a common paradigm (a knob for volume) there's no learning required. The steering wheel also is already a busy place - you have two or more stalks nearby, sometimes cruise control is part of the steering wheel buttons and even if not the amount of space means you are forced to provide a subset of functions. A stereo with tactile-marked buttons and knobs is known to most drivers via other paradigms and is easy to swap between cars.