[FernetMenta]

So the Pi uses the same codec for both DVD and Live TV (hardware acceleration - and you can chose between mmal and omx routes in the hardware acceleration menu) - but the Core doesn't. Is this because am-mpeg2 doesn't work with VOB content or because the Pi, unusually, has switched away from ff-mpeg2video for DVD playback?

With v17 all codecs capable of draining (squeeze out the last picture from decoder) can use hw acceleration for DVDs. That is VAAPI, DXVA, VDPAU, VTB, MMAL to my knowledge. Those are also the hw decoders that can handle DASH without noticeable video stutter when resolution changes mid stream.