If you have low SAMe niacin can be helpful because it mops up excess methyl groups that you can't use (until your SAMe levels are restored). So it isn't always just for overmethylators.
If you have low SAMe, niacin will lower it even further, since it is one of it's main functions is as a methyl donor.
https://academic.oup.com/ajcn/article/76/5/1151S/4824259