If you think MSG is fine, try eating a tablespoon of it with nothing else.

Then another time try eating a bunch of mushrooms or similar by themselves with a similar quantity.

Likely not the same feeling. Digestion and absorption of food changes with how you consume it. Eating a sweet potato or a bunch of sugar doesn't hit the bloodstream the same way.

In my teens and twenties I found I got migraine headaches after eating certain foods; with a little experimentation I found foods (all kinds, not just Chinese) containing added MSG gave me a headache within 2-4 hours. Eliminating those foods eliminated the headaches. Later in life I found I was less effected though with sufficient quantities I could still get one. It was pretty clear the only food that affected me this way had MSG in it. Most people seem not to be affected.

Personally I don't mind if people add it to foods as long as it's labeled honestly so I can choose to avoid it. No different than with peanut allergies or sodium content or trans-fats. Adding yeast extract or hydrolyzed vegetable protein (proteins broken down into amino salts including the glutamate) to foods to avoid mentioning you did it for the MSG is the only thing that bugs me. It's like saying a food is low-salt because you added Natural Ocean Water instead of salt.