Be very careful when you meld a materia to an item. If you see the message, "This equipment's item level is not high enough to gain the full benefit of this materia" or something along those lines, then that piece of gear is already capped or near capped on that stat. If you look, it will also tell you how much you actually get by attaching that materia. If it says +0, then you gain no benefit at all. If it says +3, then you gained most of the benefit.

Combat gear stats work exactly the same way. If you try to meld STR to a piece of MNK gear, most likely it will not give you any benefits as STR is already capped on that piece of gear. The difference with crafting gear is that it only involves three stats (Craftsmanship, Control, and CP), and for most pieces of gear it's possible to meld enough materia to reach the caps for all three stats. You need to be very careful, though, that you aren't melding more of a given stat than that piece of gear can handle.