As for sleeving barrels with softsolders, they have plenty of shear strength for the job. Think about it. Multiply the area of the solder joint times the shear strength of the chosen solder. There are fairly small forces trying to push a tube out of a sleeving job or monoblock. Many fine guns are still built with monoblock softsoldered tubes.

I'm betting the reason lead/tin solders were chosen originally was for their ease of use in a small craftsman's shop. This not only includes the soldering process itself, but the cleanup prep for finishing as well. Try cleaning up excess braze (pick one; silver, bronze, etc.) a fillet in some thin barrel/rib area that is brazed. You'll likely mess up the tube wallthickness and be cursing before you're done.