Williams doesn't actually handle the ball very much. He is essentially a poor man's Thaddeus Young. It bears noting that Young's defensive activity is the only reason he gets away with his flaws. Young's athleticism stands out at PF but at SF he is just another guy (and the same can be said for Williams).
Williams Play Data
19% Transition: 1.04 PPP (37th Percentile)
19% Spotup: 0.90 PPP (44th Percentile)
14% Cut: 1.36 PPP (80th Percentile)
11% Isolation: 0.98 PPP (78th Percentile)
9% PnR Roll: 0.82 PPP (16th Percentile)
9% Post: 0.91 PPP (66th Percentile)
5% Off-Screen: 0.94 PPP
4% Handoff: 1.00 PPP
4% Putback: 1.21 PPP
4% Misc: 0.94 PPP
3% PnR Ball: 1.00 PPP
Only 23% of Williams' offense is On-Ball Half-court offense (Post, Iso, PnR).
Conversely, Melo is at 58%, so there is clearly a wide gap between Williams and players who actually create their own shot with regularity (read: more than a one-game sample).
He moves well off the ball, but is terrible in the PnR due to his poor shooting. He also isn't the Fastbreak finisher one would think from watching a highlight video that only shows made baskets and not the missed shots and turnovers.
One can argue a higher Vertical rating maybe 10 points higher, but 82 Strength is already quite high and it's not like his speed is any better than your average wing.