I hereby title this post "Ingesting supplemental levels of glutamate — the single most important endogenous neurochemical — invariably does various stuff to the nervous system, the single most complicated system we study. Spending decades debating between black/white positions of 'it's always bad for everyone' and 'it's always fine for everyone' is ridiculous."
I've never met anyone in real life, or had a proper 1-on-1 online with anyone (with basic biology/neuroscience knowledge) who denied MSG's health/biological effects.
It's frickin glutamate. That's it. GABA is the main inhibitory neurotransmitter. Glutamate is the main excitatory neurotransmitter. Ingest various/increasing doses of the former, and you will be relaxed verging on sedating. Ingest increasing doses of the latter and you (and your whole nervous system) will be stimulated verging on seizure-y (for various reasons it wouldn't be possible to orally ingest enough glutamate to actually seize).
It's all a matter of doses. But seriously, grab some size 0 capsules, put 500mg of GABA or Glutamate (sodium glutamate aka MSG being the most readily available) in each, mix 'em up and have a party. You'll easily be able to tell which one you've taken after 15-30 mins. This is basic stuff.
Individual traits (species, morphology, and brain chemistry/epigenetics) all play a part, the same way they do for sensitivity/tolerance to any other ingested food constituent, herb, supplement, or drug. But for Glutamate at least, these factors mostly equate to differences in tolerated doses, because in the end there isn't anybody (or any species that I know of) for which Glutamate doesn't play the vital nervous system role we know it to play.
So, that's basic biology. Supplementing Glutamate/MSG generally isn't needed/wanted/pleasant. But is it so much of a problem in the amounts found in food? That's what the whole debate is about after all.
The answer is there is no answer. It's a nearly meaningless question. It's the kind of meaningless question/debate that comes out of the relationship between regulatory agencies and business/industry, because the former is tasked with the impossible task of making conservative black/white/one-sized-fit-all health decisions for the public, and the latter is tasked with making addictively-tasty things regardless of health impact as long as it's legal. They NEED the answer, and the answer they've come up with is SAFE.
The real answer, like any real answer to any biological/health question, is it's complicated. Or at least variable. It's a matter of how much you ingest until you experience/feel something you don't like. Maybe it's 500mg, maybe it's a gram or two. It doesn't really matter. "Consumers" should simply be able to know in advance the same way they do for salt and sugar content.
Please no one jump on this post for lack of sources. I'm not going to copy/paste 5 decades worth of research on Glutamate in humans/mammals for the sake of winning what is basically a nonsensical political/conspiracy-hunting-mob debate.