Matrix is definitely expanding. I often think that they are expanding faster than they can keep up with themselves, but those who have been here longer than I have tell me that this aspect of Matrix is a definite improvement over what the situation was several years ago.
I have no experience working with any of their competitors but it is definitely different than your typical primary care job! Going into peoples' homes has been a real eye opener for me! But they are also working on methods to keep their field NPs safer -- I think they are working on methods for an NP to call for help if the situation becomes unsafe.