Many of the Dover books I have used over the years I have found to be less than rigorously edited (at least to modern standards) presumably because they are simply reprints of earlier editions. Some of the Bach I have used didn’t have BWV numbers for each piece, and some of the left hand was in tenor clef! I’ll leave that to the Viola players!
Dover’s ‘complete’ Widor Symphonies also is inaccurate since, helpfully, Widor continued to edit his work AFTER publication, so there are different versions of some of the works! However, John R. Near has researched this as part of his doctoral studies and has published a complete critical edition of the 10 symphonies:
http://www.areditions.com/rr/rrn/n011_020.html
Still a bit pricey though, but at least your getting a good quality of thoroughly researched editing!
The Franck edition by Dover has served me well. Contrary to the comments here, it is rather sturdy and even after many years of use is still in one piece! You do need to read the Rollin Smith books though as there are one or two errors to beware of! The Guilmant sonatas are produced well (reprints of Schott editions) although it is infuriating why Dover should only choose to print the first 5! Schott have the remainder though.
The complete organ music of Vierne is also available quite reasonably at:
http://www.carus-verlag.com/index.php3?BLink=Vierne
As for the rest, yes it is quite expensive, though some of it is completely worth the investment - others quite simple isn't!