A question on the optimization... (mobile 3D game)

In mobile games you should keep your draw calls (set pass calls) as low as possible. Every seperate material requires a seperate setpass call. The main point of using an atlas texture is to reduce the drawcalls. Unity will automatically batch objects with the same material together if possible and draw them in one go.

However using an atlas requires you / Unity to do some additional setup work. For example 3d models have to adjust their unwrap (UV map) so it's using the right portion of the texture.

If you're able to fill all the space of your atlas texture, there's no other disadvantage. Though if you pack textures of different sizes into one atlas, there's usually some "wasted" space. However that's usually neglectible.

ps: textures are two dimensional data so they have a width and height. If you have 4 256x256 textures you need one 512x512 texture to pack them together. A 1024x1024 texture could include 16 textures of the size 256x256.