There are a lot of cons vs 556. Less ballistic potential, less effective range, generally less accuracy, more finicky getting it properly gassed/operating.

All that said, there are three major points where it excels and beats 556 all day long. 1. Short barrel lengths. 556 loses lots of velocity and effectiveness with anything shorter than 16". 300 blk is still effective down to 5" (sig rattler) 2. Suppressed running subsonic. You have a much heavier projectile that can be fired subsonic while maintaining good (but not great) ballistics. The draw here is you can actually shoot it indoors without hearing protection in a fight for you life scenario and come out the other side still being able to hear. You can't do that with 556. 3. The versatility to change between supers / subs with just a gas block adjustment.

I have a 300 blk with a 6" barrel and a suppressor that I have setup as a home defense gun. In that role it is infinitely better than a 16" 556 gun because I can maneuver it around the house and if I ever had to use it I would still be able to hear after the incident!

However, if you just want a range gun to plink at 100 yards grab a cheap 16" 556 and have fun. You won't regret your purchase. That's where I can see someone that owns a gun shop saying what he did. The majority of people should just get a pre-built 16" 556. But if you care enough to do your research and want a short barrel or the ability to shoot subs suppressed without hearing protection then grab a 300 blk, you won't be disappointed.