Watched some Durant all-22 (Philly, Detroit, both Seattle regular season games, Chicago and Seattle in the playoffs) and he's basically as advertised, a good, solid starter, but obviously not elite.
Long story short: Very good zone feel and plays with good leverage knowing where his help is, good ball skills, reliable open field tackler, lacks elite twitch and size and can get boxed out against bigger bodies. Very comfortable with him as a CB3, can probably function well enough as a CB2 but will give up some plays against better competition (and will make his fair share too).
Short story long: Because he's not elite he'll give up some plays to elite WR's (Brown, Smith-Njigba, etc.) but what I noticed is that he very rarely gets beat by secondary type weapons like Kupp, Shaheed and Odunze. He's small so he can get boxed out at times in tight quarters by bigger bodies, but the times I saw it felt flukey (like the ridiculous 4th down TD to Kmet against Chicago that tied the game that should've been offensive PI).
Aside from that nonsense 4th down play his performance against Chicago was probably one of the best individual performances you'll see out of a CB. Multiple PD's, including one at the goal-line to save a TD, 2 picks with nearly a 3rd, and really good open field tackling.
I can see why the Rams did what they did at CB though because their entire secondary had a rough NFCC and were arguably the reason they lost the game. With that said, they locked up Seattle's passing game pretty good the first two times, and sometimes it just goes against you, especially playing a division opponent for a 3rd time. In the two regular season games Darnold was 51/78 for 549 yards (7 YPA) for 2 TD's and 6 INT's, their defense pretty significantly outplayed the Seattle offense across 8 quarters in the regular season (until the extremely flukey last 10 minutes or so of the second game) and I think it was probably a matter of Seattle's offense finally figuring out how to attack them schematically after getting knocked around for two games.
I think they could've easily won it all with a guy like Durant starting but they wanted to aggressively go for it before Stafford falls off a cliff and CB was really the only position that lacked top end talent. They're returning basically every starter on both sides of the ball, with an elite WR duo, a very good RB, one of the better OL's in the league that has played together for a while now, and one of the better front 7's in the league. The only spot where they were kind of just getting by was at CB, so that's where they went all in going into possibly Stafford's last year.