Is there any problem with unconnected faces in the same mesh?

Hello, I hope I cam be clear. Let’s say I’m creating a tree in blender, and I want to add some big branches. I can extrude, or I can make the branches as a separate object, and then just combine the objects. Now, this way, although now is the same object(and same mesh), not all faces/vertices are connected.

Is there any disadvantage on letting the mesh like this?
Image:

Because it seems much easier if I want to add a lot of branches and it even saves me vertices.

Both will import just fine. It’s really a matter of what works best in the given situation. For instance, in your example the connected branch will be easier to texture without creating a visible seam where the branch intersects the trunk.

There are a lot of cases where unconnected vertices are useful as well though (foliage billboards, for instance) It really just depends on your specific use cases. Hope that helps!

I personally tend to model my branches unconnected so that I can duplicate and move them around in blender to create the whole tree.