Hardware video decoding is handled in AMLogic chipsets by a co-processor called the VPU (Video Engine (AVE-10)) - Video processing unit
There are no problems with Hardware video decoding on the S912.
The GPU co-processor is independent of the VPU. The GPU and its drivers draws the Graphic User Interface (GUI). This GUI can overlay on top of whatever video the VPU is decoding in the background.
kszaq's S912 LibreELEC releases prior to LE 8.1.7 had Kodi user interface screen tearing due the less than optimal Android GPU video drivers being used. They were to slow to draw the whole GUI without noticeable tearing. Those GPU drivers have now been optimised to fix screen tearing.
Then you have the octa(8) core CPU itself in the S912.
Add the CPU + GPU + VPU together and a few other bit and bobs and you and up with what is called a System on a Chip aka SoC.
All those core's added together in the SoC produce significant heat. Chipset cooling and heat dissipation is more important than ever on S912 devices.