The results on the ski are what count. You have to consider that ski service is like dealing with a contractor. You can tell them the results you are looking for but don't question their methods. You don't really need to know how they do it as long as they do a job that is good and up to your expectations.
Leaving aside places like SkiMD, if you want it done right, do it yourself. Not only is it ultimately better for your wallet, you'll also save time carting skis back and forth, plus the pick up drop off interaction time filling out forms or waiting on line, but you'll also learn what you like, KNOW 100% WHAT THE TUNER JUST DID TO YOU when the skis feel funny, and you'll be able to control exactly the areas where you increase the base bevel or detune or increase the size bevel. Because until you are doing it yourself, you are always at the mercy of the tuner, his/her INTERPRETATION of your instructions and needs, and whether you left them a six pack.
You haven't seen some of the results of home tunes that I've seen. You'd be amazed at what an home tuner can do to their bevels. Don't get me started on (not) flat bases.
Apart from grinding the base, can a person at home tune a ski and really get the edge angles correct so that it compares to a job that is done by an experienced shop with specialized machines? My gut feeling is that it's unlikely. If I'm mistaken and it can be achieved with practice and patience then I would be up for trying.
Contrary to my remarks above, absolutely. Except for the bases. There is no equal to a good stone grind.
As to how to select a shop, walk in without your gear and ask about their tunes. Ask to see a pair of skis ready for customer pickup. Is the base shiny and wax free? Are the edges sharp? Bring your true bar and see if the bases are flat, edge to edge. If any of these criteria fail, move on. If they can get these things right, bring in your skis and give them a go.
What a shop looks like in the back room doesn't necessarily reflect the quality of the work they do. Our shop is a bit dusty, very cramped and not perfectly organized. We
do put out quality work. And don't think that just because a tuner is young, he can't do the job right. Our new tuner is 24ish and meticulous. I'll concede he is probably the exception, but you can't judge a book...
Automation isn't a problem either. The operator is far more important than the type of equipment although maintenance of the equipment is important.