I said I would not trust a canned tune without checking the AFR on the dyno. = For my peace of mind. I did not say I would not run a canned tune and I think they are great for most consumers.
You don't need a dyno to check A/F ratios. In fact, if you were referring to the tailpipe sniffer they put in cars that would show a skewed number depending on whether the car has cats or not. If you have an SCT and a laptop you can check your own actual and commanded a/f ratio driving around on the street(safely of course!).
Beyond that, A/F ratios are only part of the picture as there is also ignition timing, camshaft timing, part throttle driveability, too many to list.
Personally, I am actually more wary of custom dyno tunes. I am lucky to have several of the top Mustang builders and tuners right here in my area but not so many people can say that. If you took your 5.0 Coyote engine to be tuned by someone who has no experience with this engine you have a much higher chance of destruction and mayhem than running an e-mail tune. So many shops say they have years of Mustang tuning experience and still go out and blow up a Coyote engine because they had never worked on one before.