There is a section in my forthcoming article on engine sounds but basically there are two types.
The cheaper ones are sound generators which use electronics to simulate the engine sound. These are adjustable for speed, tickover and cylinders but are NOT real engine sounds.
The ACTion P100, on the other hand, is a digital sample player in that it holds a recording of a real engine and uses complex electronics to vary its "speed" along with the throttle commands.
In terms of being a "real" sound there is no comparison – unless you actually want a clapped-out Dennis bus.
Like Paul T says you only get what you pay for. However the P100, along with its amplifier and speaker, is still a lot less than any other comparable unit on the market e.g. Benedini, Graupner, Model Sounds Inc. If you want the sound of two turbo-diesel engines then you can forget the "simulator" type of sound unit – and I don't make a penny from selling ACTion stuff these days!
Dave M