The warning about taking folic acid without also taking B12 has been around for 30 or 40 years (yes, I'm that old). Folic acid has the ability to mask a concurent B12 deficiency.
I think that the real problem is that folic acid can block receptor sites at the BBB and that is why we try to limit them.
Personally, I tested high for homocysteine even though I was on 1400 mcg of folate and 1000 mcg of cyanocobalamin daily x 4 years. I was in thyroid and adrenal failure and just beginning the long decent into ME/CFS. If the Dr knew something about methylation and/or the symptoms of ME he would have saved me from a couple of years of hell.
But to answer your question....it depends on the dose you plan on taking, your (genetic) ability to convert folic acid to mfolate (aka something useful), and too many other variables to accurately predict your response. But many of us choose not to go there.