If you step into a 737 you don't know what you don't know. Maybe the on board computer needs to be in mode X with setting Y etc.
I thought about this for a bit, but generally stand by my position. I know that the certified airliner is required to not require unusual pilot skill or attention to fly. So a non trained pilot would really struggle, and likely not display much competence with the advanced systems.
But, for a normal operating jetliner, your average PPL could keep it under control in conditions they were used to (VMC), they would just be behind with speed management, and planning descents etc.
The point is that the post PPL confidence will be largely type specific - they could be quite confident in one, but very uneasy in another. I know many C 182 pilots who would be really not confident jumping into a C 18, but that does not take away from their being confident in the 182.