Don't guess and speculate about Foster's performance. Here's a very good fact and videotape-based analysis of him.
https://www.profootballfocus.com/new...-reuben-foster
He had a tremendous season last year, then this year before his problems in SF he was disappointing. But you do need to watch how well he can cover the intermediate part of the field when the secondary retreats into a Cover 3. That's something we do frequently to limit big plays but TEs like Ertz slaughter us because of it. Watch the clips of Foster covering Tavon Austin on the intermediate crossing patterns.
The speed is what separates him from Mason Foster, and the instincts in coverage are what separates him from Zach Brown. He didn't play well this year though, so it's fair to wonder what we're getting from a performance standpoint. At the same time, the tape don't lie it's clear to see why the Skins loved the idea of scooping him up.