Grant is listed as 6'-6". In general, that is too small to play PF in the NBA and way too small to play C. Some players through strength, length of arms, or general athleticism can overcome that and be successful as a PF or C at that height. Grant only to a point.
Grant is a decent second tier off the bench player. He has some position versatility, can hit a 3 from time to time, and generally plays to not hurt the team. That is not bad for a 22 pick, probably average or a little below average for the draft position.
I don't think there is anything wrong with Grant Williams. This is just what he is. If there is anything "wrong" it's that the weaknesses in our roster force us to use him in roles he is not suited for, such as starting or being the first big off the bench.