Yeah from what I've read about Williams, he's more of a jump ball specialist that doesn't get great separation, and that PFF separation chart definitely aligns with that. That being said, he was also very good in a small sample size last year and Herbet loved throwing to him. It's entirely possible that he was just a bad fit with Rodgers in NY.
That being said, Williams is a guy you bring in to be a WR3 at best right now. I'm more comfortable with their WR depth with Williams at WR3 and Austin at WR4 than Austin at WR3 and Van Jefferson at WR4, but they still need to add another legitimate WR2. He's a lottery ticket that is more of an upgrade on Van Jefferson than an actual solution to the WR issues.