This has been a topic of discussion many times previously, and something I had personal interest in since I have a KitchenAid that sees a lot of use.
The general consensus (and I'm sure someone will chime in to correct me) is that they just aren't very good at it, and the crush you get from a KA grain mill attachment isn't appropriate for brewing. I would go even further to say that there really isn't a lot of overlap in terms of mills that can both crush grains for brewing, as well as making a good fine grind for flour. Maybe some expensive commercial mills, but for the hobbyist level I haven't really found anything.
I ended up getting a Barley Crusher a couple years ago...maybe 3? And it has been very solid. I know some people here on HBT have had issues with theirs, but I haven't experienced any of them so I don't really have a personal opinion beyond "it works great for me".