/r/ is used for an approximant in English, by a number of well respected phoneticians including Daniel Gimson, John C. Wells, and Peter Ladefoged. As long as the discussions defines what is meant by /r/, that's perfectly acceptable, and as long as only English is under discussion, it's not ambiguous. But if both English and Spanish are under discussion, of course you don't want to use /r/ for both English red and Spanish rojo, because that would be very confusing. It's the same with Irish and Hungarian  as long as only Irish or only Hungarian is under discussion, there's nothing wrong with using  for their respective palatal stops, however different they may be. But if you're discussing both Irish and Hungarian, of course you'll want to be more specific about their relative places of articulation. +gr