Branthwaite is most importantly a rare physical profile for me. Big, tall, strong, but still has very good pace and mobility. I think just this rare combination, similar to Hojlund, makes him a very good prospect for the future, but he's also someone who's already good enough to massively contribute from day 1.
His box defending also improved a lot last season at Everton IMO, as defending deep for large portions of every game was a big change up from what he was used to, in a more dominant team at PSV in the Eredivisie. Box defending was one of Varane's biggest strengths when he played for United, but in my opinion he was not good enough to play in a high line. With Branthwaite, he's perfectly suited for that whilst also being on the course to becoming an excellent box defender. He would be a long-term project, just like Yoro, but the Englishman is a further ahead in his development and in my opinion he's ready to play most of a whole campaign at Manchester United at 22, unlike Leny, who will need to be slowly phased in for the first 1-2 seasons, due to having much less experience (50+ Ligue games at 18 is still very impressive, mind you) and being 3 years younger.