Well, Doom Classic for the iPhone is a decent enough exception. It’s not perfect, but there seems to be a control scheme that at least approximates the feel of a keyboard and mouse. If you have an iPhone or an iPod Touch, like Doom and have $7 to spare, there are certainly worse apps you could buy.
The difficulty select is very well disguised on the menu screen, though – I played through the whole of episode 1 on Normal without realising that difficulty select was even an option.