Nowadays, the degree of hard-edge treatment can be a matter of an artist's personal preference or perhaps is demanded by an important class of viewers. For instance, some fans of aviation or railroad art might prefer to find rivets and sheet metal joins crisply and correctly shown and there are artists temperamentally inclined to produce such illustrations who will do the job. I'd say that it's the artist's wishes that usually prevail, because the greatest part of his work falls in an identifiable zone on the hard-edge to painterly continuum.
I tend to favor painterly art, but thought it might be worthwhile to present some examples of detail-oriented artists who specialize in science-fiction and fantasy art.
Vallejo and Bell are married and sometimes work together, as for the painting above, or make illustrations on their own. Biographical information on Vallejo is here and for Bell here.
Palumbo is Julie Bell's son from a previous marriage, as mentioned here.
As this indicates, he has been called Brom most of his life, and it does make for a nice, easily-remembered brand name.
Biographical information on Jusko is here.
I don't have a title for this Palencar work.
No Wikipedia entry as yet, but this is what Vitale has to say about himself on his Web site.
The same applies for Baga. He mentions that for a while he was studio assistant to Donato Giancola, an established fantasy artist.
Illustration for fantasy and science fiction should be given more than casual consideration because it represents one of the few remnants of illustration art as it was practiced before 1970. Incidently, much SFF illustration is make using digital media these days, but the artists mentioned above prefer traditional painting.