Windows 10 works very well in a new computer today (or new since the summer of 2015). But for older computers, especially ones that originally ran Windows 7, you can potentially run into some big problems. The big problem is that, although the hardware drivers for Windows 7 and Windows 8.1 are supposed to work for Windows 10, they don't always. And, for an older computer (like one running Windows 7 originally), there will most likely NOT be any driver updates to help it run Windows 10 properly. So, if you have an older machine, you may, if you upgrade to Windows 10, end up with a machine that no longer works properly. It may be that the laptop DVD drive is no longer recognized, or the wireless adapter in the computer is no longer recognized, or worse.
So, by best advice is:
DO NOT UPGRADE TO WINDOWS 10 unless the computer was brand new at the end of the summer of 2015, or more recently.