Your hardware probably has buggy ACPI tables.
This is the most likely cause. Windows being more stable than Linux is laughable at best. While Windows doesn't crash as often as it used to, it still crashes from time to time; and that's even with every software and hardware vendor in the world supporting it.
The last time Linux had a crashing problem was in 2008 when Foxconn intentionally programmed their motherboards' ACPI tables to detect and crash Linux. They denied it, but a deep dive into the ACPI tables strongly supported the notion of intentional sabotage.
Windows, on the other hand, crashed regularly when everything was working as designed.
With that said, Kubuntu's GUI does have stability problems. However, killing and restarting plasma-shell has always resolved them.