Sorry, video acceleration and codecs are not my area. But I can give a bit of background.
One of the principle limitations on Ubuntu Core at present is that graphical snaps need to include any userspace graphics libraries corresponding to the kernel drivers and the graphics hardware.
For the Mir based snaps we’ve chosen to include the Mesa userspace libraries. These open source libraries provide support for a wide range of hardware and have a stable interface to the kernel drivers.
For Intel based graphics I’d expect that staging
va-driver-all will provide support for video acceleration. But there my knowledge ends.
The situation on ARM is different and typically each SoC needs a bespoke kernel module (which, unless specific enablement has been done, may not be in Ubuntu Core) and a bespoke userspace.
In particular, the proprietary Broadcom DispmanX drivers needed for the RPi3 are not enabled by the default Ubuntu Core kernel, and nor is the corresponding userspace included in any of the Mir snaps.