Back when people didn't have the disposable money we have had in recent history, and when these machines were new, they usually had one machine. And they'd do whatever needed done on that one machine. Many folks used FWs for things they were not designed to do, like sew leather and got away with it. But as has been said the FW is small, has a low power motor with little real torque, and is just generally not made for leather.
Our little machine is a 48 vintage, I believe, and sews well. I have no intention of ruining it and would sew nothing heavier than suede or denim with it.
For heavier leather, get a 66, or a 15-91 or a 201-2.
Joe