Some mod motors can be unreliable. I have seen some onroad cranks done by one of the BIG names in motor building that looked amazing and ran like nothing else but they also broke within a gallon. Of course these were not the cranks he sold to the public, those are almost stock.
If you go too far on the timing or screw up the top surface of the intake ports (short circuiting the engine as some call it) then it will be hard to tune and drink tons of fuel.
A motor that's done right should last at least as long as a stock motor and run better.