There is another CON to using Quick Synthesis, though it is not a factor for someone level 50 it is a factor for anyone still leveling up : it sharply cuts the experience earned.

If you go to make an item, and just Synthesis it without adding any Quality at all, lets say you get 950 experience for it.
If you then go to make that same item, and using entirely NQ materials, you get it to 100% .. you get a 300% bonus, for a grand total of 3800 experience for it.
If you Quick Synthesis that item, you get 425 experience each.

That, right there, is the primary CON to using Quick Synthesis when you are 49 or lower.