Most, not all, 5.56mm is poor quality brass compared to the better commercial products. Even where it is 'good', ie consistent weights, wall and neck circumferences around the case, properly annealed necks and shoulders, it has a lower capacity compared to Winchester, Norma, and Lapua 223. In such a small capacity case, that's generally a bad thing, reducing usable maximum charges and hence velocities.
However, there is another thing one really has to watch with ex-military 5.56. The Nato spec is very high pressure indeed in the standard 62gn bullet loading, nearly 62,000 psi. That is a punishing level. And gas-powered semi-autos are HARD on brass - they are whipped into and out of chambers in weapons whose headspace adjustments would sometimes give a civilian rifle builder a fit or two. Fired cases with their heads (ie bases) forced out of square to the cartridge axis aren't unknown. Massive cuts into the case-head / rim are often seen, this caused by the case hitting a peg ejector at speeds you couldn't approach in a manually operated firearm, and dents in the case-mouth / shoulder received by the case hitting the edge of the ejection port are also pretty common.
Then as noted primers are crimped into the pocket (to stop them backing out and causing a malfunction when fired in an automatic weapon with excessive headspace). This has to be reamed out after sizing / depriming before the case can be re-primed, or the entire pocket swaged.
So, ex-military brass can be good and reloadable, but it is a matter of being very careful as to what it is, where it came from, and looking hard at its condition. Generally, you get what you pay for things and the people who make military ammo churn it out in the millions at the very low prices that miserly governments will pay for it. The surprise isn't that there is bad military brass around, rather that some of it is as good as it is. If you can find once-fired Lithuanian manufactured GGG brass that has been fired in civilian rifles for sale, it has a good reputation.