Here's why Windows 8 is underrated:
Lack of a traditional Start menu; VERY BASIC functions like "shutdown" is hidden under the "Settings" charm
Microsoft developers saw an opportunity to disregard legacy and make the kind of radical changes that only a brand new OS offers. They needless eliminated the traditional Start menu and made VERY BASIC computing functions like "shutdown" an adventure to find. As a result, a cottage-industry of 3rd party Start menu programs like Classic Shell and StartIsBack is released. In every earlier versions of Windows, the way to shutdown is Start > Shut down, but Windows 8 hidden a very basic computing functions like that. In Windows 8.1, Microsoft added the power options to Win+X Menu but still many people were having troubles in finding those options.
No direct boot into desktop
When Microsoft introduced Start Screen in Windows 8, many users complained about the direct boot to Start Screen feature. People wanted to direct boot into Windows Desktop as they used to do in earlier versions of Windows. After getting many requests, Microsoft added a new option to enable direct boot into Desktop in Windows 8.1. But the option was not enabled by default and the operating system was still booting directly to Start Screen.
Can't put up with changes
People can't put up with change or such in general and hated Windows 8 because it was different to what Windows had been for almost 20 years since Windows 95 (as of Windows 8.x era)
Adding to the aboveThe bleeding obvious of course is that the fundamental design of START and its menu structure has remained unchanged since the venerable Windows 95 and this is because it’s beautifully simple and just works.
Windows 8.1 brought back the Start button that user miss to make the desktop a primary user interface instead of a Metro app.
Windows 8.1 Update 1 fixed nearly all the problems.
Windows 8.1 Update 1 adds much requested Power and Search buttons to Start Screen. Now people will have no problem in finding options to shut down, restart or hibernate their computers in Windows 8.1 Update. You're able to use these power options directly and easily via Start Screen.
Windows 10 finally brought back the Start menu.
If there's any other reasons - I don't have a lot of time right now to make this post best I can - feel free to state them.