What problem will I encounter while still using Unity in win7.

A1: As you're probably aware. Each machine is unique so it's impossible to predict any problems that you may encounter. Win10 has been out for some time now, so it has matured a bit, there shouldn't be any problems, but impossible to say.

A2: There is nothing wrong with continuing to use Win7, at least none that I'm aware of .

I have noticed some older programs working on Windows 7, but not Windows 8/10. None of these were made with Unity, however I think as long as you are sticking with Windows 7 as a minimum you shouldn't have any issues. Some of the problems with using any OS older than Win 7 is having the proper VC Redistributable, .NET packages, DirectX versions, and possibly older drivers that don't work correctly with modern graphics/sound functionality. You might have issues if you try developing with WinVista or older, but other than that you should be okay.