Oh, really? Have a look at this thread and note especially what Pat Rogers and DocGKR (aka Dr. Gary Roberts) have to say:
http://tacticalforums.com/ubb/Forum1/HTML/001022.html
A search on m4carbine.net indicates mixed results with the M4:
http://www.m4carbine.net/archive/index.php/t-2541.html
http://www.m4carbine.net/archive/index.php/t-30469.html
Semi-auto shotguns can work very well for hunting or playing games, but they still come up a bit short as combat weapons (this is the black and green shotguns forum, right?).
Apples and oranges, it sounds like those folks are primarily looking for something to employ less lethal and breaching rounds with, not as primary defensive arm. Most of the problems with tactical semi autos, assuming a good recent design, stem from being used with reduced recoil buck or birdshot, when run with full power combat loads they are more real world reliable than pumps once you figure in short stroking under stress, and increasingly, malfunctions due to bad build quality in what are now seen as budget guns.