Here is my personal experience.
I had a client of mine getextensions by a girl who "does nails too"...She thought she got a bargain cos she only paid £160 for the hair and the fitting. Great, but then she had to pay me £30 to cut them in for her as the girl could do no cutting AT ALL.
So then, four weeks later, she says how her hair feels really dirty and could she come for a wash and blow dry - no problem. In she came and the extensions (micro rings) were all bunched at the root and hair was out and there were bald patches on her scalp. The girl who fitted them had told her they had just slipped (in fairness, about 5 of them had slipped) and she would move them up for her.
I sent a photo of her hair to another geek, who is very experienced in extensions, who said GET THEM OUT! I washed my clients hair, who was going to go back tot he extensionist that evening, and once I had washed it, the extensions matted into one massive lump. I have never ever seen anything like this before. I had to remove them for her.
This is just my experience, but from my clients point of view, she went to someone who knows nothing about hair, and 4 weeks later had to have them removed.