Lots of things to like about him. Big body, excellent speed both over short and long distances, good movement and work-rate, confident with the ball at his feet, holds up the ball, places his finishes, scores headers, seems passionate and driven. 3 areas of concern for me:
1. First touch – that "mistakes" video had some horrible touches, but having watched him more, that concern has been mostly alleviated. I want to show you this recent goal against Porto that was disallowed for a bullshit "shirt sleeve" offside, so you won't see it in any compilations (click to watch on Youtube, start at 1:20):
A striker that can bring the ball down under pressure like that and create a clean shot doesn't have a problem with the first touch. Having watched that game, it's also remarkable that this was pretty much the only chance he got all game – similar to the games against us he seems to be able to concentrate on executing one big moment even if he's starved for service the rest of the game.
2. Passing – that's a much bigger problem for me. A playmaking striker he is not; in fact I think his passing might be even worse than Mane. Typically for a player who always relied on superior acceleration and athleticism, he didn't seem develop that side of his game at all. We should see some improvement with good coaching, but that will never be his strength.
3. Injury concern – Darwin already had a couple of knee injuries in his career and for a player with such a big body and swashbuckling style, a possibility of more injuries will always be a serious concern (he's similar to Haaland in that BTW). He definitely won't be as durable as Mane or Mo have been for us.