If a roofer is taking shortcuts I guess you could say he or she is actually a con man or woman. The act is terrible because you are not doing what you were paid to do because the home owner has no idea that maybe you may be tacking a short cut. The first and biggest issue is the roofer not reading the specs to the roof and may start day one with major issues
Not placing underlay is a common practise. Please bare in mind that in the eighties is was common practise to skip on this practise because it was legal. Now days skipping on the underlay not only cause short term major problems you are definitely not covered in any warranty.
under ventilating a roof is a cheap form of saving money. If a roofer is under ventilating your roof he is planning to disappear in the short term. Your roof will have problems with in a year and you wont figure out why until its to late. Make sure you notice all the outlets you have on your roof so that if you suddenly are missing one cause the roofer covered the old outlet with roof. A bathroom fan that is suddenly ventilating into the attic will cause long term damage to the home and you will be paying the price.
Gable flashing is needed when you are have a cedar conversion. Gable flashing protects the deck from rot when the rains hit the plywood creating mould and rot to occur. Gable flashing is easy to not do cause it is something that is not there when you have a cedar roof. So when you get a new duroid roof from the cedar roof you had before you should notice the string of metal running down the gable board.
Dumping the debris. It is not so common but a regular occurrence when the roofer will notice that the home owner has probably never climbed into their attic and will decide to save a little money dumping the old roof into the attic. This is an issue when there is no access to the attic or crawl space and only the roofer ever knows until five years pass and you are replacing your roof due to such high condensation levels the deck is rotten and your walls are green with mould. The situation when a vaulted ceiling is stuffed with old roofing shakes and shingles blocked the vapour barrier and suddenly you have trouble. This normally happens when you hire a crew you had a bad feeling about anyway.