We constantly hear horror stories about customers that have been ripped off by phone repair shops, the stories are all so similar "the shop has totally screwed my Phone up the touch ID no longer works" or the screen is coming away. I have known people to be told that the touch ID will no longer work after the screen has been replaced, always ask the shop this question if the answer is "it will no longer work" then run! The only way the touch ID will stop functioning is if the home button it is totally destroyed.
Another answer you may hear when complaining that the screen is coming away "this is normal after a screen repair" This is absolute nonsense!
These are the first signs the shop /guy has setup to make a quick buck after watching a few youtube videos.  
There is also the issue of under cutting genuine Phone repair specialist by offering ridiculously low prices, of courses why would you go elsewhere "that guy that I used is really cheap" he's also the busiest repair shop locally,
Because he/she is the cheapest of course why wouldn't you go there? Well if it sounds too good to be true then chances are it probably is.
They try to repair customers phones as fast as they can with no regard for your precious device, they put screws back in the wrong places (a big no no with iPhone) or don't even put them back if they are unsure.
The screen replacements are also non original mostly cheap poor quality replacements.
Would you ask someone to re-wire your home without knowing if they were qualified?
Never be afraid to ask to see proof of qualifications after all its your device that you have worked hard to purchase.