It's not that his strokes are bad, but looking at him, at a guy that tall, with good coordination and movement, people expect him to be better.
I think his tennis mind is pretty good. He's no Nadal but he's not Zverev either, who I think plays like a lot of traditional Russian players (rally from the baseline all day, Doesn't seem to bring a lot of instinct or creativity).
Serve- It's a 7/10. But he's tall and well coordinated. But his motion doesn't seem to get his whole body into the serve. He kinda stands where he is and rotates as opposed to really leaning into the serve.
Forehand - 8/10 but not really any scarier than anyone in the top 5-8. One thing I notice is that his forehand has a hitch to it and he tends to arm it too much. On about a 1/3 of his forehands, he sticks out the right arm and whacks it. He's not using the left arm to bring the racquet around, getting the fluid natural rotation that incorporates the whole body.
Backhand - 6/10. Decent motion but imparts too much spin and it's not the heavy kind. It's just a rally shot.
Volleys - 7/10. He comes up with good volleys and his hands are pretty good. He's still figuring out how, when and at what angles to come in on but I like that he's one of the few young guys that likes to finish the point coming forward. I think he can really end up with as a real good all court player. Sorta like Fed. I know, I know, Fed's not an all court player in the Sampras mould but it's as close to one these days.
Movement- 7-8/10. He moves well, especially for that tall of a player. He's no djokovic but more like a Safin.
Returns - like everyone says, it's not good. His swing is too big but the results aren't scary. If you're gonna take a big swing, the results better be like Gonzo or Safin. There has to be a power/consistency trade-off but he's not getting either right now.
Power - Here's where probably the main gripe about him is. He's not totally a power player. Even his forehand, which is arguably his best shot, requires 2-3 shots to start getting an advantage. He really can access more power.