A character made of cubes? Is it bad for gaming? I've seen these models on sketchfab that I find good enough for a mobile game, but if you look at them in wireframe mode it looks like they are made of a succession of cubes (or very close shapes).

I'm far from an expert at modeling, but isn't it a very bad practice?

In blender, what's the difference between creating multiple objects and a single object composed of multiple meshes?

How will it impact rigging / animation?

If I would import them into a game engine (unity for example), can it make a difference (in performance/usability/shading) ?

In short, are these models bad?

Thanks for your help.
The BGE needs more time to process multiple objects. Therefore it is advised to join objects that form a larger part of the landscape (e.g. a forest).

On a character the requirements of the character handling will dominate. 

From artistic perspective an object that deforms (skin) is better to handle as a single object and animated via armature. 

If you have objects with rigid parts like a robot or a car where nothing deforms you might use multiple objects. Nevertheless the armature method is still applicable.

You might even need a mix of both. E.g. the deformable mesh for presentation and the rigid objects for collision detection (check if the hand touched anything).

This strongly depends on your needs and your focus.

Other requirements might be:


breakable (can be done with both, but different objects are easier to handle).
lots of instances (here you need to look at the performance).


Edit:

When you want to change items and clothes in-game you better have them separated. If that is not the requirement they can be part of the skin mesh.