[Brightcolours]

When you have mechanical manual focus, you can use that instead of AF if you want to , for macro stuff. But when you have focus by wire that is not excellently implemented, manual focus is no joy at all. So yeah, a reason to skip suck a lens if it has AF issues (or is it just slow?) and not so good manual focus by wire.
And you say you are curious about that, but he lays out the issues pretty clearly:

1. Manual focus is non linear
   
2. focus angle depends also on the speed with which you turn the focus ring ("Like most I think that this behavior makes focusing manually harder")
   
3. Another issue is that sometimes the focus change isn’t continuous but happens in jumps
   
4. Add the small lag experienced with all focus by wire lenses and overall I found the Tamron to be harder to focus manually than most other E-mount lenses.
   

All this means that you keep overshooting focus and that it will be tedious to shoot manual.