Regarding dragging luggage, I've found it's a nuisance to get through the turnstiles for RER and Metro but this was not a problem for AF bus. You just need to keep an eye on your stuff until the workers load it for you.

Regarding stairs, at T2 I did not have to go up or down stairs to get to the AF bus, but it seems we did pick up other people at the lower level at CDG so it may be necessary depending on which terminal you've arrived at or are departing from.

I did not find the seats on the AF bus to be any more narrow than the RER or metro. A taxi would offer a bit more space, of course.

The biggest drawback in my view to the AF bus is also a strength: each route includes very few stops. If you're going from CDG to areas close by Place de l'Etoile, Porte Maillot, Gare Montparnasse or Gare de Lyon then you're in luck. If not then you need to figure out how to connect via some other means such as Metro or just skip AF bus altogether.