The issue is that the sync system brings substats down to their maximum allowable levels. So where at the time a i345 weapon with melds could have 280 primary sub stat and 278 secondary, if you sync down you can reach 280-249, made up for by being able to hit i345 in other slots (vitality is the main concern in ultimate anyway). Being able to meld on top of that would be nuts.

If materia worked on sync, you could use a crafted piece to have 280-279-92, a whole 93 substat points over what's supposed to be allowed. Play that out over twelve gear slots, and you'd have a pretty crazy ~5% or more damage boost.