I don't want to put Manuel Caldas' (in Spanish) shoes on, of course! He's, after all, the foremost authority in all Valiant and Foster related things (him and Brian M. Kane, I guess). If you don't read Spanish, I'll just sum up his opinion saying that Manuel thought that Fantagraphics used the best possible material to do a definitive Prince Valiant edition, but didn't. Among other things he mentioned the small size of the books and how poorly the material was scanned (by Syracuse U., it seems, not by Fanta) blurring the art. In his own Cerebus TV, Dave Sim says more or less the same thing while comparing the reprinted pages with old newspaper pages.
That's what I'll do below too adding what, to me, is the biggest problem in these reprints: color saturation.
Hal Foster (et al), "Prince Valiant" Sunday page # 726, January 7, 1951 (l); the same panel as published in Prince Valiant # 8, Fantagraphics, January 2014 (r).
As you can see above the colors are a lot less saturated on the newspaper page. The result is a lot lighter; so, the drawing isn't blurred and overwhelmed by the coloring. (The out of register red on the character's lips is another story.)
Hal Foster (et al), "Prince Valiant" Sunday page # 504, October 6, 1946 (l); the same panel as published in Prince Valiant # 5, Fantagraphics, April 2012 (r).
In the example above we can see how Prince Valiant's cape became a formless blot. Notice also how some lines blurred and bleed until becoming (or almost becoming) black surfaces (the stick on the left, the shadows on the snow).
Hal Foster (et al), "Prince Valiant" Sunday page # 1016, July 29, 1956 (l); the same panel as published in Prince Valiant # 10, Fantagraphics, January 2015 (r).
The example above is the more telling of the three, methinks. The Fanta panel is so heavy that the drawing details disappear almost completely under the saturated color.
Hal Foster (et al), "Prince Valiant" Sunday page # 470, February 2, 1946 as published in Prince Valiant # 5, Fantagraphics, April 2012 (l) and Tarzan # 6, NBM, 1994 (r).
I'm comparing above the Fanta edition with a page published in one of the Tarzan volumes published by NBM. Do I need to say more?
All in all, it's a shame that Kim Thompson didn't accept Manuel Caldas' offer to work in this collection. With a little increase in size it would definitely be the definitive Prince Valiant reprint. No doubt about it.