iPads and tablets get destroyed/stolen in a lot of plants. Then you have to worry about battery life, charging, charging stations, network range (in/out conditions), etc.
Certifications, drop test, temperature ratings, Estop, pressure switch to enable/disable motion... Normal tablets cover none of those.
As soon as you try to solve those problems with a rugged tablet you add weight and cost. And battery life is a problem if you want a full shift.
I'm not trying to talk you out of wireless, I just haven't seen a rugged wireless tablet that is worth using.
Keba has a few devices specifically for machine teaching and axis control. T10 or T20.
Yes, they are wired. But that gives you an e-stop, keeps it "tied" to the right machine, saves weight (batteries) and gets rid of charging issues.
http://www.keba.com/en/industrial-automation/ketop-operation-monitoring/mobile-operating-panels/ketop-t20-eco-techno/
There are others out there too, but that's the best one I've seen.