You were not wrong in what you said. Three phase locos in general are supposed to be more reliable than normal ones.
But P7 and G9 suffer from some bad design issues because of which they are giving over-heating issues. And also IR's conventional locos have been really perfected over the years because of which they are also very highly reliable and are able to compete and hold their own against the three phasers. The problem is somewhat specific to P7. WAP5 on the other hand doesn't face such issues.