I have been using MTBC for several years, so far not too many complaints. Of course, they do some minor mistakes sometimes, like missing modifiers which I find and correct by looking at EOBs, or calling the insurance (my receptionist does it). I don't think I should expect 100% collection and outstanding work from somebody who only charges me 4%. I realize, that an experienced in-house billing person would collect more money than MTBC or any other billing company. How much more would they collect? How much more would I have to spend for the billing person salary and billing expences? That's a question.
My billing is very straightforward. I submit CPT/ICD codes to MTBC, and I make sure medical necessity is met before I submit them. So far it works.
I used to work for a big group and we paid 10% for billing/collections. My collections were worse then, than now.