While in theory using your phone or tablet makes sense, it misses a crucial part of the experience: knobs.
If you've ever tried e.g Fracpad, or operating a QC or Hotone without ever using the knobs...it kinda sucks. Not having real, physical controls you can turn sucks. Is it usable? Sure, but it just sucks as a user experience.
If modelers offered either a proprietary knob controller, or even just sensible MIDI knob mapping features (context based editing) then it could work. You'd put your tablet wherever you want, and a small knob controller near it. But nothing on the market supports this in a way where it makes sense.
Until that happens, I'd rather have a dedicated touchscreen on the unit, plus knobs.