In many years of flying, I have taught a lot of pilots, both military and civilian, of every race, colour, creed and of both genders. I can truthfully say that every single stude has had the same treatment and level of instruction, ie the best that I could give.
I have to admit that at times it can be difficult, mostly if you suspect the stude is not trying as hard as he/she can but in the long term it makes no difference.
Regrets? Maybe with hindsight having put so much effort into training studes (when the RAF did train a lot of overseas pilots) who then turn on your own country in later years.