I wanted to know the reasons why people don't prefer them....
Because not custom.
Here's an interesting fact about generic maps - Apart from a few extreme climate countries, every single Golf GTI and ED30 in the world had the same generic map when it left the factory
Bluefin Superchips, Revo, APR et al, they all do the same thing. They take a decent, healthy car, map it on the dyno, tune it on the road until they are happy, then sell it as an upgrade. It is no different to what the factory did. Although the factory obviously has access to extremely expensive industry standard DIN measuring & tuning equipment, plus a huge team of ECU & software engineers. Therefore you can guarantee consistency across all the cars, which is a benefit of generic mapping, especially where strict emissions targets are concerned.
I don't like this term 'generic' map because it implies lesser quality to a custom map. All of the names mentioned above produce good results and are within 10-15hp of each other, that's where things can start to get bitchy. HP chasers only focus on one thing, so I would stay away from the Facebook group if you don't like childish mud slinging
These pre-written maps are safe. Nothing wrong with that, because the vendor can't guarantee a customer's engine is in perfect health, so they err on the side of caution. You wouldn't believe some of the bodges some customers turn up to mappers with. Wires hand twisted together, MAFs unplugged or missing, boost hoses that pop off because of cheap crappy clamps, intercoolers hanging off the front with cable ties etc etc. It's exactly these kind of people who slag off the tuners for giving them low numbers. Anyway....
Custom maps push things harder because the tuner is 'live' monitoring all the key sensors and can see how the engine reacts to his inputs. This gives a bit more scope for power & torque.
Superchips have been in the game a long time and most people are happy with the results. I had a Revo map in mine when I bought it and it drove really well! Super punchy and smooth.
If you like how the car feels after you've flashed it, then it's done the trick. Can you squeeze a bit more power out? Of course you can, but when you've got a safe 250 to begin with, another 10-15 is not going to blow your skirt up.