Yes, it is mild steel.
The only issues they have as far as the cases go is that steel doesn't expand like brass so sometimes more carbon buildup forms in the chamber during firing. Sometimes this can cause shells to get stuck, especially if switching between steel and brass. A quick swab and scrub removes the buildup.
As far as the extractor goes, some say its harder on them...but considering you are spending $3-4 less on 20 of these than on cheap brass...after a few boxes the savings alone will pay for a few extractors.
The only hard evidence that I've seen that this stuff is worse for your firearms had more to do with the projectile. Often steel cased ammo has a mild steel core projectile washed in copper. I saw a study where this can lead to greater barrel erosion than using standard lead core. But those results were noted at over 10k rounds.
Do the math...
10k Tula/Herters : $3,000
10k average brass: $ 4,500-5,000
That $1.5-$2k you save can buy a few barrels...or a new rifle.
Some other issues are that some find it low on the power scale, so if your AR is sensitive to weaker ammo then performance may suffer.