A vast number of games and demos fail with it. 95% of hw-banging games and even more among demos.
There is nothing wrong with letting AmigaOS bootup.
There is nothing wrong with using the OS to read the keyboard.
There is nothing wrong with using the OS to read data from a device. (floppy, hard drive, CD, DVD, whatever).
Commodore made them a great OS and gave it away free with every computer.
If the coder refused to use it then its his own dam fault.
You can always let the OS load and then do a Forbid(); Permit(); around your main loop if you want to "disable" the OS temporarily.
You can temporarily disable the Interrupts too.
There is not much reason to throw the whole entire OS in the garbage when writing a game or Demo.