ORIGINAL: Von Ohain
IC motors is bad for high altitude because the air is thinner, and the carburettor of model IC motors isn't sophisticated enough to compensate.
So your motor will stop due to thin air even before the model is out of visible range (been there done that).
Notice that full scale planes with piston engines and carburettor has a mixture knob just as easily acessible as the throttle knob.
Pretty sure Maynard Hill cracked the problem on this. I know he set an official altitude record with an IC engine that was around 8k feet and made flights over 10k ft.