I don't think so. Synthetic sea water is pretty well made. You may be a stickler for the minute details though. If thats the case then natural sea water is the way to go for you. Keep in mind, if you use products to treat it you are really just squewing it to be more like bought salt. Natural sea water will not have the alkalinity or quite the calcium levels normally recommended for an aquarium though.
There is a reason we choose to be a little higher in these respects than natural sea water. Most salts for reef tanks (oceanic, reef crystals, red sea coral pro, sea chem reef, etc.) address both these issues and make them higher. Almost any salt (anything known to have lower calcium levels which isn't as big a deal in a FO tank) is going to still have a higher alkalinity than NSW.
I doubt you will find anyone in the forums that thinks you cannot have an extremely successful reef or FO tank with synthetic sea water though.