The amount of work put into a map will depend on the spec of the engine, a stock car will not require a lot of changes from your 'pre determined' changes that you have made.
Saying that, its unlikely that the car will be perfect without further changes to the map. Some customers will use 95 ron, others 98, some want a more progressive power delivery, others want a very responsive aggressive map.
Generic maps are ready made, used by tuners that will just match a hardware number and flash it on with a 'that'll do, it was ok on the last car it was used on' attitude. They might have 5-6 ready made software versions that are just re-used time and time again. We often see cars with software that contains a completely different hardware and software number from that on the ECU when it left the factory, many cars with other cars chassis numbers, some cars with software from different model versions i.e pd150 stock maps copied and pasted into a PD115 ecu.
A generic map can be good, it can be pants. Its buyers beware to a certain extent, but at the very least the tuner should be checking the work by datalogging/dyno testing. the amount of cars we have seen over the years that have had cheap flash and hope remaps that have caused issues is scary, tfsi's with timing pull of 8-9, diesels with massive injection demands, diesel files with only fuel pressure raised and nothing else, BMW's with the rpm limiter set at 10,000 rpm.....
Generic diesel tuning is acceptable to an extent, but should still always be checked and not just flashed onto the ECU and driven away.
The problem is that even the worst of the worst tuners using cloned tools and map files are claiming to be offering the highest quality custom maps for £100, ideally the industry should be regulated in some way.