Agreed that they are not all bad, but how do you tell the good from the bad? In most industries there are certifications that provide some information (not enough, but some). Apple will certify technicians for computers, Microsoft will provide certification tests and certifications, Oracle, IBM, Google all have comprehensive certification programs. There are no certification programs for independent repairers or repair shops.
Apple currently offers iOS certifications only for Apple Authorized Service Provider technicians (AASPs), and they can be found only in countries with no or few Apple stores. But the reason is that Apple does not consider iOS devices to be user-serviceable; except for battery and screen replacements Apple's own stores don't service iPhones, iPod Touch's and iPads (or Apple TVs). It is simply more economical to centralize repair in a few locations where labor costs are low. For screen replacements Apple has a "robot" that does all of the work; put the phone in one end and a phone with a new screen comes out the other end (not literally, but close). No independent has anything equivalent. But you are correct, that Apple will no longer void the warranty on a 3rd party screen replacement if nothing else was damaged in the repair process.
So how do you do research? While you can use personal recommendations, they are limited by the fact that most people don't know a good from a bad, unless it is really bad. You can't go by review sites such as Yelp, as its reviews are so heavily stacked they are worse than useless. (Yelp is not along; Trip Advisor recently gave top ratings to a non-existent restaurant).
If the independent repair industry wants to be taken seriously it needs to develop meaningful certification programs.