My little un is taught by a Filipino in Annuban 3, previously in A1 / A2 she was taught by Thais, the difference in this semester is f8cking huge. Yes, sometimes she comes home and says "Papa, teacher May say Penciled not pencil" but WTF, I say 'nowt' rather than 'nothing'.
My accent is rural Cheshire (UK), and if I were to speak to my students naturally (how do, all reet? = hello, how are you?) they would have f8ck all idea what I'm talking about, but perhaps more importantly is the fact that 99.9% of them will never meet another person from Cheshire, so learning 'Cheshire' English would accomplish what excatly?
So Filipina teachers don't know a truck load of English idioms, so what, when the f8ck would the average student get to use English idioms? "Somchai, I need to chew this one over with you" is not really gonna be used that much. Filipina teachers do know everyday 'basic' conversation, which is probably far more useful for most Thais.
I'd say Filipino teachers are fine for government school students, the majority of whom will not need advanced (or even basic) English skills once they finish school. International school students should be receiving education from teachers who are qualified from whatever the country the curriculum is based, if that includes Filipinos, fine.
Personally I understand most Filipinos better than Scots, Irish, Welsh, Cocknies, Brummies, Liverpudlians, Mancs, Geordies, Macums, and folk from the West Country, doesn't mean to say they're better teachers.
And, TMU, my daughter's teacher is as fit as they come

RR, here here.