The problem is, chainmail doesn't block the impact. Distributes the force of the blow, but doesn't block it. Like a thick sweater that's resistant to cuts.
Against something like a razorblade or broken bottle = works great. Hammer, baseball bat, thrown brick = pretty worthless.
In the middle ages, knights would wear an inch of padding under chain, but those German cops in the pictures don't seem to.
Obviously it doesn't do a whole lot against firearms, although double chainmail with welded rings reportedly stops 9*18 (presumably still results in broken ribs).
Oh, and it's heavy as balls, to the tune of 40-60 pounds on a full hauberk.
Overall, plastic riot armor would be a far better idea.