I used to teach PPLs coming from GPS bashing to start CPL training. It was a nightmare! Like teaching someone from scratch all over again. No idea whatsoever of how to dead reckon properly, they had just been used to following 'the line', so the accurate flying that was required just went straight out the window. No idea how to do a proper diversion, ETA etc etc! This inevitably resulted in more time to complete the syllabus, costing them more money in the process. The ones who used their chart, clock and eyes always found the course easier, as they had the spare mental capacity to cope with the rest of it.