Is it better to assign different materials to different parts of the mesh and then tile textures on those materials, or is it better to UV map, unwrap and then create the texture this way?
Of course I won’t unwrap models with a lot of round parts and other complex geometry, because it would be painfully annoying, but let’s say I have a moderately complex model.
You should read the guidelines for Unity art asset submissions. Generally meshes that have not been unwrapped is considered incomplete. As stated in the guidelines assets that appear to have their UV’s automatically unwrapped will also be rejected. I am curious of why you “of course” would not unwrap round parts? If the mesh is made well, unwrapping it should be simple enough.
The “painfully annoying” bit of properly and efficiently unwrapping your mesh is what purchasers are paying their money for. I’d be furious if I bought a character model from the asset store that had used separate materials for the body parts with tiled textures for “skin”, “cloth”, say.
Personally, I’d expect a maximum of one material per object, but preferably less - if there are several objects that are likely to be rendered at the same time, they should share a material. And @pengocat is absolutely right - you should read the asset submission guidelines.