When did your dentist graduate? In my limited experience, older dentists are far more cautious of composite materials than they ought to be.
New composites run the range from being extremely flowable and minimally filled (such as sealants really) to being highly packed and experiencing minimal shrinkage.
In my experience it's much easier to completely fill a prep, after eliminating decay, with composite than with amalgam. I'm simply more comfortable flowing a thin layer of flowable composite into the prep, then layering it with packable composite. Much of the success of composite, especially in large load-bearing teeth in the posterior, is a result of operator technique, but placing a good composite that will withstand function is far from impossible and quite manageable.
I've seen composites that leak, or fail at the margins, with the result being recurring decay under the filling--but I've also seen the same with amalgam.
From the phrasing of your question it sounds as though the dentist believes that the materials in composite somehow aid the decay in growing moreso than the materials in amalgam, which is untrue.
And as far as failing restorations and leakage goes, well, I've spoken about that above. There really are some amazing things that can be done with composite these days, and the future will continue to bring a larger shift away from amalgam, both from the clinician's standpoint and as the patients continue to request "white fillings".
