He Struggles Against Competition
Agent Block also has a problem with stepping outside of his paint to guard more mobile centers, for example, Karl-Anthony Towns.
Hassan looks as if he is terrified to move away from his beloved paint to guard his opponent.
Whiteside is not quick whatsoever. He moves slowly and therefore he is incapable of leaving the paint.
Here is an example of Whiteside getting cooked by a more mobile center:
Although this is a video from the preseason, you can still compare Whiteside and Embiid.
In the video, you can observe two more things about Whiteside: how he performs against a good defender (Embiid) vs a mediocre defender (Okafor) and his length.
Whenever Agent Block is playing against a good center (DeMarcus Cousins, DeAndre Jordan, Marc Gasol) he does not play very well. He is timid against these centers.
When he plays against a low-tier center he looks like he is having fun on the court. However, against a top-tier center, he looks as if he is ready for the next game before the game even starts.
Hassan still competes against them, but since he cannot impose his will as easily against them, he mentally checks out of the game.
He cannot guard centers one-on-one. His man defense is very deficient. He is just tossed around like a rag doll on the court. It is embarrassing to watch.
Now, let me step off of my soapbox to discuss what he is good at.