World modeling: unique mesh or different pieces?

Hi guys, this is my first thread on this forum. I'm a 3d modeler for some years now, I've modeled something for games but anything important.
Now I'm going to model a world for an open world game and I had a doubt, the terrain should be a unique mesh or different pieces?(For example street as a series of planes and the sidewalk as another object...)
I put it in the getting started section because I think it's a quite basic thing to know

 

The choice is up to you, and your personal workflow. However, each separate Mesh accounts for one draw call. Meshes that can be combined should be. Unity batches meshes below a certain size, but the size is very low.

https://docs.unity3d.com/Manual/DrawCallBatching.html

You also have the UV's to worry about, so a road could either be painted right onto the ground or made in geometry. Each approach has its advantages.