They are a tiny POD with no distributor (Ingram wholesales their books) and no marketing outside of online sites. That doesn't make for book sales and that should concern any writer considering this publisher or any other one similar to them.
I have no idea what their contract looks like or how they pay royalties, but after reading the other thread where they posted replies, I have little confidence they know what they are doing.
No, I'm not an author. However I do handle royalties and some other administrative aspects. McCarren has a very generous royalty structure and in fact is very selective when it comes to authors because of this. If you do any search on the fourth book, "Do The Devil's Work For Him: How To Make It In The Music Industry (And Stay In It)" by Amy Sciarretto and Rick Florino you'll see quite a presence. Amy Sciarretto has a 15 plus year music journalism career and with an admirable following.
Which basically tells me (if I'm reading this correctly) that these authors are doing well because of their established following. A new author would have no following, so where exactly is the benefit to that new writer if there is no distribution or marketing?
As a personal note: I don't find any of the covers at all appealing, they look very amateurish. And the website design hurts my eyes a bit, the colours are a bit too contrasting. A more subtler blend than the dark dark blue and the beige would probably make the effect a lot more pleasant.