The problems with owner-training:
1) Low success rate. An inexperienced trainers odds of correctly selecting a candidate are 1:100.
2) What to do if the dog washes out--do you rehome? keep the dog? can you afford another?
3) Inexperienced trainers tend not to have the skill to realize when their dog is not suited to the work or not sufficiently trained.
Contrary to popular myths, owner-training is neither faster nor less expensive than going through a good program.
All of that said, is it possible to owner-train a good service dog? Sure. I've done it. But I apprenticed under an experienced trainer for a year to learn the craft. Not everyone who sets out to learn to train dogs, no matter how hard they try, will be able to. I stink at chemistry. I studied really really hard, but I still stink. It doesn't make me stupid or lazy. It just makes me a bad chemist. (Good thing I'm an engineer instead) If a person has never trained an advanced dog before, and I'm not talking about pets but real working dogs like SAR dogs, gun dogs, or stock dogs), then their odds are slim. If a person has at least put an AKC CD on a dog, their odds are more promising.
Program dogs are not inherently better than owner-trained dogs by a long shot. I've put my owner-trained dog up against some program dogs and mine was clearly the better trained dog. There are good programs as well as bad. I've seen some bad program dogs. Unfortunately, most of the bad dogs out there are owner-trained. Hence the bad reputation of OT dogs. Most of the stellar dogs are also owner-trained, but trained by folks with a lot of experience.