I don't disagree with the statement regarding BM doubles versus a BM single. SM however, puts the heavy stuff on the bottom and the floaty stuff on the top, so it offers some natural stability advantages over backmount singles and is significantly better than back mount doubles, where the heavy stuff is on top of the floaty stuff and is more inclined to try to swap places.Funny I see people in backmount doubles who're struggling with the basics like trim and bouyancy all the time. I tell them to go back to a single tank, work on the basics. Give it 20 dives in their more familar configuration before adding the doubles again. They almost universally agree (after the fact) that pushing into doubles too soon was a PITA that slowed down their learning curve.
And while more mass in involved in either case, resulting in a larger bubble in the wing that has to be managed and anticipated more than with singles, it's very easy to start out in sidemount with smaller/less massive tanks like AL 80s that are also less negative, reducing the larger bubble management issues that otherwise come with doubles. Given the increased comfort and stability, a diver is, IMHO, less likely to struggle in SM than in BM doubles.