I can attest to the durability of the canvas versions, I've used one for a couple of years now and it's still in great shape. They will hold and conceal just about any size handgun, but as others have mentioned the real constraint is weight if you will be wearing the coat for long periods of time. You mentioned using it for a J frame and I can't imagine having any issues at all with this jacket carrying one regardless of whether it's a steel or alloy frame. Regarding larger pieces, mine regularly sees a S&W 457 with no issues regarding weight or printing.

When I got the jacket some friends were interested in buying them as well and we did some experimentation to see what could work. A large frame Glock (20/21) is doable for me. A steel frame commander concealed well, but weight becomes an issue after a few hours. A 4" 629, done to test the extreme limits of what works, conceals but really is too heavy to be comfortable.