The generation of new chunks is incredibly slow and generates many lagspikes, on the client side as well as on the server side.

It is the same thing when you want to teleport: horrible lagspikes and chunks that have trouble loading (example: video 4).
Note: This applies to both new chunks and already generated chunks.

Plus, mobs appear before the chunks load / generate.
So, some of the mobs die (example: video 2 with a suffocating guardian).