The tablet environment isn't particularly CAD friendly, Windows nor Android. Either it's the tiny market segment, or just as platforms they are just aren't that friendly input or graphics-wise. The few that I've tried in the Android environments were hopeless, even by comparison to Autocad or Datacad twenty years ago. The viewers are adequate, but to do sketching or field work nada, zilch, nothing.
Graph paper and a clipboard still rules for armchair sketching or in-field detailing...then shoved through the scanner for .pdf conversion and distribution.