If I could afford one, I'd consider it.
The problem is there aren't any codes that affect the smog test. There's a code for the NOX sensor, but that doesn't affect the smog test.
It's the INC's that are coming up for the catalytic converter and an O2 sensor. With incompletes the smog tester will not pass my vehicle. That's where I started on this little venture - by going to the smog test place.
A few years ago the B&C had some issues with his 88 Pathfinder. A shop in town wanted to do all kinds of work they said it required in order to pass smog. They estimated around $1000 or more. He couldn't afford that much, so he called our trusty mechanic and he did some work, just enough to pass smog, not enough to make a mortgage payment. Then he sent us to a local shop that does just smog tests. Passed with no problems.
Our mechanic isn't trying to get rich off his clients, so I trust him.