He doesn't suck. He is a talented player with some clear limitations one sided skillset for wing position.
Don't listen too much about Dortmund fans, fans tend to exaggerate heavily about any player who struggled or been semi flop, like they have zero redeeming qualities afterwards.
One key factor to consider is they signed him as a 9 (what he was in Salzburg), when that didn't work out they shoe horned him unto new wing position. That takes adjusting.
Fast wing forward that attack reasonably well can work out in new setting, Lookman was seen as a flop in every club he was in before turning it around with Atalanta.
But there's two key things that worry me about Adeyemi (because his one sided qualities can be gameplanned for in specific tactical role). First is his health, and second is if he is worth the big price Dortmund would ask. If it's reasonable fee its ok. But the injury track record remains a worry.