Yes, if it's running, and all the stuff is working correctly, the MCU, EEC (depending on engine) work great. When they don't work correctly it can get frustrating.
The manuals for testing various parts, and procedures is invaluable. Most will tell you how to go about fixing certain engine diagnostic codes, in a step by step routine. This helps to prevent replacing parts on a vehicle that are still good.
Getting parts for them is a nightmare sometimes too.
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As for rural counties being smog test exempt... It goes by zip-code. Usually in counties with high elevations, that the factory smog equipment would not be compliant at. My truck for example is only compliant to 3500 ft.
You still have the obligation to keep the smog related equipment in good working order. The CHP can pull you over for smog related violations and make you fix it to California specs no matter how old it is. Smoking engine, rich running etc... could get you nabbed as example.