It is worth pointing out that back-action locks were very often used, by some of the best makers, when building a target rifle with an exceptionally heavy barrel, or a heavy barreled Plains rifle. The reason for this being that there is no problem with clearance for the mainspring, as there would be with a conventional "front action" lock.
There is a great deal of difference as far as aesthetics, from one maker to another, when it came to building a rifle around a back action lock. Some maker's rifles were pretty ugly, while others could make the overall architecture quite pleasing.
As far as supporting the drum on a rifle with a back action lock, I have three original rifles and an original percussion shotgun that have an extension on the front of the lockplate for that very purpose. It not only supports the drum, but the drum helps stabilize the lockplate in its mortise. Of course this was a moot point on guns having a patent breech.