On the plus side, he's rapid and has a good shot on him. In and around the box his unorthodox style can be beneficial, and he's tall enough to generate panic at dead balls.
Against that, his game without the ball is middling at best, and he is pretty poor with his back to goal. Part of that is not having the game intelligence to know what run to make that best helps his team mates, part of it is what might charitably be called lack of application - if the ball is so much as 3 yards either side of him, don't expect him to move to get it.
For us, Gozie Ugwu was miles better when it came to play the number 9 role, and even he was fairly average. Duku got some goals early doors, but some of that comes down to the fact no one got to grips with us at the start of the season, and we found ourselves playing in acres of space.
In the end, he's a bit of a curate's egg. Definitely has good attributes about him and I wonder if he wouldn't be well suited to a second striker role feeding off of a proper number 9 and crosses into the box in a 4-4-2. However, in terms of a more modern forward role I think he's badly lacking tbh. None of us were sad to see him go, and it's telling that Ugwu, despite less goals overall and less appearances would have been the one of the two centre forwards who weren't loanees that we would have kept.