How many HMIs do you need?
You won't get nice HMI graphics on your phone, unless those have been specifically designed for the phone.
The pixel/physical resolution on a phone is just too small to show you a normal HMI screen; one that was designed for PC use.
Tablets are better suited than a phone to view HMI screens. In the odd case, they sometimes have a smaller physical screen than a PC, yet they have a higher resolution. Think Surface Pro with a 12" display but 2736x1824 resolution!
It's something to think about when using HMI screens designed for a desktop PC.
An HMI with scalable vector graphics (SVG) can help out there, but does not completely solve the problem.
Those are just a couple issues with wireless, mobile HMIs.
If you are going to deploy 10 or 20 HMIs, or it's mission critical then you'll want to pay big $$ from the top vendors.
If you just need a couple HMIs for quick access, then there are many options (ex.Try VNC to an HMI computer).