I guess I would never trust online mastering in a generic sense. I think that the danger is that mastering is then viewed more as a process rather than being an art, which I happen to believe it to be.
As a process it becomes too much about the tools and what is done in a logistical sense, IE just making a track louder, fit a certain eq profile or whatever.
As an art form it is about presenting a collection of songs as a whole and making sure they hang together and flow as an experience.
A great quote I recently heard from well respected audiophile mastering engineer Stephen Marsh, is that "Mastering is the completion of the process of mixing." He views them as part of the same process and I really think that is true.