I think folate or SAMe are primarily effective if the body has a deficit of them, either by not eating enough (folate), or having a problem with it's metabolism.
Too much folate can be dangerous as others have pointed out -> cancer.
Folate should mainly play a role in mental problems because it is needed to make tyrosine out of phenylalanine, and l-dopa out of tyrosine. It's also, together with SAMe needed to produce adrenalin from noradrenalin.
Also, both are needed to produce melatonin.
Those pathways alone could cause a lot of problems, if they are not working. I am sure they are not the only pathways, but pretty important ones for mental problems.
SAMe however is very expensive, which makes me wonder if supplementing with a sulfur donor (methionine) and a methyl donor (MSM) would be an alternative.
Tyrosine / l-dopa supplementation massively increases sulfur requirements, dependent on dosage. So people supplementing with them would deplete their sulfur levels, and in the process lower SAMe, as well as decreasing the effectiveness of those amino acids, because the catecholamine metabolism would be inhibited at those points where SAMe plays a role.
Also, folate is important in homocysteine metabolism. It can for example convert homocysteine back into methionine which then could be converted into SAMe. Basically folate makes sure(amongst other things) that homocysteine does not accumulate, and recylces it so it can be used in the sulfur cyle, which also glutathione is dependent on.
So reducing homocysteine is good for health in general, and increases the supply of sulfur for other processes.
My oppinion? Make sure you get enough folate, b6 and b12 to keep homocysteine low.
Supplement with a sulfur donor if you also take phenylalanine, l-dopa or tyrosine.
(the body should be able to convert methionine into cysteine and vice versa, so your options are those two amino acids)
Supplement with a methyl donor such as MSM.(Might be enough of a sulfur donor, if you do not supplement with catecholamine precursors, since it does also work as a sulfur donor, but additional steps are required to synthesize cysteine or methionine which are needed for other processes that want sulfur - so basically, less efficient, with a better side effect profile, and my choice if there is no massive sulfur requirement)
That way, you should have as much SAMe as your body is able and willing to produce. If that is not enough, experiment with additional SAMe, in ADDITION to what I mentioned before, to save money. Also, this will result in a better health profile by actively reducing homocysteine.
If both folates and SAMe's anti depressive mechanisms are through their respective functions in the synthesis of neurotransmitters mentioned above, their effectiveness may be limited to the amount of neurotransmitters that your body is willing to produce under the best of circumstances.
If that is not enough, other strategies to increase neurotransmitters further may be needed(serotonin / catecholamine precursors, which make a whole stack necessary to work properly, or reuptake inhibitors, or mao inhibitors), or other ways to fight depression(cAMP increase for example). Their effective dosage should be lower though, if you optimized your neurotransmitter metabolism.