You've already identified the problems with fillers, some don't wet sand, some don't adhere well, some crack or shrink, just a mix of issues.
If I find an area that needs to be filled, I mostly rely on plastic strip stock or sprue bits, shaped to best fill the area, then file and sand to finish. I only use the fillers to cure very minor spots, then either PPP or Tamiya white filler in the squeeze tube are my most used.
Larger gaps are best filled by plastic shaped to fit, since the join will be stronger than by using customary fillers, such as the wing to fuselage fit. As was mentioned cyano works well with no shrinkage, but comes with other issues of it's own.
I think the auto body fillers, (Bondo, etc,) are favoured by many modelers, since they don't seem to have the shrinking, cracking problems. I have used them and they do work well, but do require more work to finish by sanding or filing, since they do cure to a very hard end result.
Patrick