This is about UMFA (unmetabolised folic acid). Like you I haven't read too much about this, but I do have some thoughts.
My main issue is that people are normally given folic acid without giving them appropriate amounts of B12. So you are of course going to end up with UMFA, and also a functional B12 deficiency, because you are creating an imbalance between 2 things that work in tandem. I am no researcher and I haven't even read all the research, it's just my personal opinion. You shouldn't take folate/folic acid without B12 and vice versa.
I'll use the case of people on regular B12 injections as an example. There are some who have been injecting for years, weekly or maybe twice/3 times weekly, who can't reduce their folic acid supplement from 5mg per day without getting symptoms. The more B12 you take, the more folic acid you need. And all of these people say they would rather feel well day to day, than worry about UMFA research articles.
That aside, the other thing to think about is tolerance. It's like B12, some people get bad detox and over- methylation symptoms from methylB12, and this can happen with methylfolate. You need to start low and build up the dose, and switch to folic acid if you can't handle it (and the methylB12ers should try hydroxoB12 instead, or cyanoB12). There's lots of interesting stuff about over- and under-methylation on MTHFR.net.
I'm not really going anywhere with this! I think bog standard 5mg folic acid is great to correct your initial deficiency, and then once you have good levels and are going to a maintenance dose try switching to methylfolate. Or by all means try methylfolate from the beginning.
I personally alternate days 5mg folic acid one day and my B-Complex the next (which has 400mcg methylfolate in it). But I have weekly injections so need a high maintenance dose. I'm still tweaking it all the time, and that's what everyone should be doing, we're all different and even on an individual basis it's a moving feast.
Oh, and for every article that you read about the dangers of high folate, you'll find another in it's favour, here's one: