- They don't reliably stop high-velocity lead-core rounds. M193 from a 20" barrel is known to readily penetrate steel rifle plates, and lead-core .223/5.56mm like M193 is literally the most common rifle threat in America. Such plates will also fall to .270 Win, .22-250, and many other common hunting cartridges.
- They're extremely heavy. 8-9 pounds with coating, on average? That's heavier than most ceramic Level IV plates that will stop every threat you're likely to encounter, absolutely reliably.
What's interesting, given this is HN, is that the technology to make steel rifle plates -- exactly like today's -- has existed since roughly the 1910s. Nobody did it, in large part because it wasn't a good idea back then. (Rifle calibers tended to be heavier -- like the German 7.92×57mm -- and medical science wasn't as advanced, which further militate against steel rifle plates.) It's still not a good idea today, especially in light of superior alternatives.