Do you have an Apple account?
Do you have a MAC?
if not,
Do you have enough sales potential to make the effort to use their services if it also means signing up to a virtual MAC server to gain access?
D2D doesn't do much for visibility true, but neither does signing up directly with Apple unless you are going to have enough sales to make it worth your while. D2D isn't the issue as far as I can see. The issue is visibility and that's down to marketing and that's down to you getting a bookbub or pushing Apple via Facebook ads.
With Google, as a short aside, does your book feature on their display pages? I think there are only a few active ones, General, Military, Soap Opera that count. If you don't then your sales are coming from small interest groups, perhaps recommends on the google forums. Again, it's down to visibility, solved again by advertising.
Nobody I know does very well on the other sites. They seem to be a consolation prize for most of us.