Tony Williams details lay out all the necessary info well.
In summary,
The bullpup design is not any more difficult to use than any "traditional" rifle, simply different and practice and use is all that is required. Mumbles about "mushy" trigger, slow magazine changes etc tend to come from folks who have never actually used them.......
The "not invented here" syndrome along with inherent conservatism (small "c" not political big "C") is the probably only substantial real reasons the US Army hasn't deigned to really try an BP design.
The BP weapon works well in combat and has better like for like ballistics, terminal velocity, down range energy retention etc than "traditional" equally sized carbines such as the M4 due to the longer barrel.
One unmentioned reason that folk like the SAS occasionally use the M4 or AK is that both are very "generic" weapons that makes it harder for an opponent to identify nationality etc of the sneaky-beakies.
Like all weapon platforms there are compromises, pluses and minuses, on the whole I believe after actual in field use that the pro's outweigh the cons but hey, I ain't the DoD........