how do aircraft engines compare to car engines for reliability generally?
This is not possible to compare.
The reason is that car engines spend most of their time at around 10% to 30% of max rated power. A typical 2 litre saloon, 130mph top speed, is running at about 30% power at 70mph.
Whereas an aircraft engine is running at 65% or maybe even 75% power continuously.
When one sees normal car engines used in real competitions e.g. rallying they tend to fall to bits really quickly. I gather the rally teams have a pile of spare engines.
Car engines have the big advantage of water cooling, which keeps temperatures under close control, but I think they are way less reliable than the old Lycos.
And they are not more efficient, either. They are much more efficient over a wide range of power settings, which is what one needs to sell cars, but if one compared constant operation at say 65%, the two would be very close.