On the whole I don't think that the speed with which some people manage to get a PPL in various sunnier countries is usually a problem.
What *may* be a problem is that they are then qualified to fly in the airspace of a country where they have no real experience - of weather, of RT, of charts, of normal etiquette at airfields, of what the sort of congested airspace around our larger cities can be like, or just of the flying culture. That's solvable of-course, a friendly instructor at a club, or some friends in a syndicate can help you through these things. But I'd strongly recommend that anybody planning on a "quickie" PPL outside the UK, but their subsequent flying to mainly be in the UK, ensures that they've factored that into their overall planning.
G