I occasionally wonder what will happen if I ever sell my deuced Mac(hina), but as far as I can tell, even a buyer with technical competence won't be able to get to any critical data.
SSDs have very significantly changed the used Mac landscape, but I haven't seen any prominently placed, substantive guidance respecting their down-the-road implications addressed to purchasers of new machines
I don't know if this answers your concerns or not, but with magnetic media there is a
ghost or residual magnetic impression that remains through several overwrites that can be read with the proper equipment and sufficient determination. Thus the three and 7 pass secure erase options. There is no residual image with solid state media. So multipass erasing is not only deleterious to SSDs, it is a pointless exercise.
APFS does not store files in adjacent data sectors thus enormously complicating any recovery effort and rumors are the data will be encrypted by default. Additionally APFS offers encryption at multiple levels and even encryption within encryption. All of that is not fully implimented in iOS 10.3, but it is definitely coming to iOS and MacOS.
