To the best of my knowledge there is no study that systematically looked at the effect of mestinon on patients with myasthenia.

In fact the use of mestinon (and similar medications) in myasthenia is based on a study on 1 patient.

For many years it was considered the ultimate medication for MG (in fact the diagnosis of MG was doubted in patients who had no response to it, and many neurologists saw a response to tensilon as enough to diagnose MG).

Some patients do not have a response to it.
Some patients even have worsening with it.
Some patients have an unpredictable response to it.

Why does it effect different muscles differently?
Why does MG effect different muscles differently?

Why are there days in which I have ptosis of my left eye (relatively good days), days in which I have ptosis of my right eye (relatively less good days) and days in which I can hardly open both eyes?

My neurologist told me about a patient who had severe generalized MG with mild ptosis of the left eye. After receiving treatment with PLEX she had significant improvement in all her symptoms, but developed significant ptosis of the right eye. Why?

But, being practical (and putting the academic discussions aside).
If you have found a medication with relatively few side-effects, which helps you function better (at least in some aspects) take it.

Don't expect it to be a miracle cure, just a good symptomatic treatment to make your life easier and better.