Flying privatly Cessna/whatever piston for a prolonged period (over 5 years let's say) often makes pilot no longer suitable for airline environment, because along with raw flying skills (good) he also getting adopted to single pilot, light aircraft, VFR etc. Very recently we had to say goodbye to FO with as much as 4000 hrs SEP (and very good background references), after struggling for 300 hours on the right seat of airliner it become clear he cannot adopt and follow CRM, SOP, performance calculations, situation awareness in congested terminal areas etc, while most of our 300 hrs TOTAL time young FOs have absolutely no problem with it.