Pentax DSLRs are usually less expensive than Nikon/Canon -- when you compare similarly-specced cameras. The D3400 and Rebel T6 you mention won't have dual control wheels, 100%-coverage pentaprism viewfinder, weather sealing, etc. like the K-70 does -- the Rebel T6 even has fewer autofocus points.
Pentax doesn't really have a model that's as stripped down or as cheap as those two; a fairer comparison would be against the D5600 or Rebel T7i.
I can't explain the image quality comments though. In a head-to-head comparison,
the K-70 looks pretty good to me. Unless maybe the complaint is about Pentax's in-camera JPEG engine. I wasn't too fond of its sharpening on my K-30, but that's a 2012 camera and I don't know how the engine has been improved since then.