Shakira,
I do not think it is an issue as long as the CC can make themselves basically understood.
I had the pleasure of chatting with some CX FAs who were waiting for their crew bus, I had no problem understanding them, all it took was some tolerance on my part to get used to their accent, after that it was fine.
In fact they spoke English better than some people I went to college with, and they were native English speakers!!!
Level of service depends entirely on the individual & their training, not on their nationality or race. I have both awful and excellent service from all sorts of people...
On a side point, I didn't realise that CX employed "non -Asians" (for lack of a better term) as CC... I assume this is what "gwailo" means- hope it is not a derogatory term, I'm guessing it's some sort of Cantonese or Chinese for westerner or white person?? Interesting...