It could be that you were missing a vital element, Sodium Nitrite. Old Mediterranean cultures where such meat preserving methods were first developed used salt that was mined from the earth, which wasn't pure and usually contained enough Sodium Nitrite to properly cure the meat. Prosciuttos today are sometimes treated with much the same blend of minerals, although the certified ones are processed using seasalt (and noticably stinkier). This practice of using seasalt only is a more recent development.

Generally speaking, curing a ham this way requires a relatively cool climate. If you were making that Serrano here in Florida and were hanging it on the porch under ambient temps, it may have been too warm, thus resulting in the off taste/aroma and the tougher texture. Hang time is also a big factor in quality; the longer it hangs the better it is supposed to get, assuming the climate conditions are favorable.