reliability in a car does not equal reliability in an aircraft. The way the engine is run in a helicopter is fundamentally different from a car (torque, operating speed, usage of full throttle for extended times)
In addition to that, there are other issues to be dealt with, such as sudden air pressure and temperature changes.
I agree that your average Lycomings are an ancient design, that should've probably been replaced (by something watercooled with overhead camshafts) about 30 years ago, but you can't really say they aren't reliable.
If you want to know how hard and
expensive it is to certify a new aircraft engine, just ask the folks at
Thielert in Germany.