Sancho has a lot of work to do on his stamina; he fades quite a bit towards the end of games, but that might be something he improves upon as he settles into his matured body. He doesn't work intently going backward, but he will usher a man into others for them to sweep up. I wouldn't say he is lacking defensively as in won't attempt to track, but he isn't some machine who can put in a defensive shift and then burst back up the pitch to continue his forward duties. His tank would be rinsed under high, two-way load.
Truth of the matter is, with an attacking line like ours, and a pair of attacking full-backs, we should be on the front foot for the majority of games in a league campaign and giving the opposition cause for concern in really opening up or trying to force us back for concerted periods of time. Bissaka and one of Fred of McTominay should be enough to contain with Sancho ushering, and more importantly, on a counter-counter, you're then getting the more open play of the Bundesliga from Sancho on our break, which we have multiple seasons' worth of evidence that he is one of the best players in the world at turning a breakdown into a goal-scoring opportunity. Tactically, there's a lot for opposing managers to think about in over-committing to drive that kind of player back.
I think any notion that Sancho just comes here and is without fault or that there shouldn't be concerns about certain elements is not being objective, but when you look at the player and the set-up objectively, nearly ever concern can be fixed and the positives to what he brings far outweigh the negatives. In an open market, with every RM/RWF available for purchase, Sancho is still a no-brainer to be right at the top of that list behind only Messi and Mbappe outright, imo. He's of the level where you bring in the players who let him express himself to the fullest, sit back and enjoy the show, as opposed to a number of other right-sided attackers who you don't make compromises for and expect them to come in and work with what's here.