Grafting a low vigor variety onto a strong rootstock will not necessarily make it grow vigorously but will usually give it the best chances for the genetics it carries. Sometimes there are incompatibilities between varieties (in some species, at least, I'm not sure about Ficus carica) which will reduce vigor. I had some challenges in succeeding with my first grafting attempts about 20 years ago or so but after getting a few tips haven't really had trouble with any grafts. Some people have told me that I'm a very good grafter but I think it's just understanding some basics and having some experience to know what to try. Garner's Grafter's Handbook is a very good resource book but understanding callus temperatures is something I don't think it covers. I need to spend some time some day to put together some good grafting tutorials on video. My attempts so far aren't of great quality and I need to work with someone to do good videos while I'm doing grafting. I have a Youtube video (just search "white sapote grafting") that was shot with an inexpensive camera and it still gives a pretty good idea despite some portions being out of focus. Some people really have trouble with white sapote grafting for some reason but mine worked out well and figs are pretty easy (easier than most, I'd say). Wait until bud break before trying and take some steps to prevent the scion from drying out (use Parafilm, Buddy-Tape, bag it, or, if in a greenhouse, keep humidity up).