I don't there is as much of a problems as is often assumed with supplements. There may be some intermediaries that buy this or that and sell it as something else, but any large manufacturer isn't going to have that problem, everything is tested from end to end to meet standards.
Another good example is people who are homozygous for MTHFR and related mutations. Synthetic folic acid isn't going to do them much good. It may work before they are born if their mother is hetero, they may be able to scrape by on dietary sources, but you aren't going to see reductions in homocysteine or elongation/reduced shortening of telos with 800mcg of synthetic folic acid in someone who can't turn it into methylfolate. It may just give them headaches (subjective, might not be a study showing this or could be from accompanying gluten if I'm a celiac in foods often enriched with synthetic folic acid, but folic acid appears to give me headaches at 400mcg+ in food or supplemental form) every time they eat it in significant quantities as it may limit the receptor uptake of methylfolate via competition (shown in a pernicious anemia study somewhere, where folic acid levels had to drop before B12 deficiency symptoms could be resolved by supplementation?).
Lots of interesting bits here:
http://www.snpedia.c...x.php/Rs1801133
At the very least, the MTHFR studies can show that a synthetic vitamin is not the same or doesn't carry the same benefits as a synthetic. Different molecule, different effects. Even in people who can convert lesser folates into methylfolate, I'm not sure every lesser folate gets converted, so where is the rest of the synthetic stuff going?