Mythtv 0.19-p9163-r1.ebuild does not enable XVMC-VLD when the video_card_via and XVMC use flags are used. This causes problems on playing live-tv within Mythtv, it causes the frontend to segfault and crash out immediately when XVMC-VIA is used as the mpeg2 decoder. The ebuild needs modifying to configure XVMC-VLD when using via card and XVMC use flag.
as per upstream, you should only pass "--enable-xvmc" which is what we're doing. So the functionality of the ebuild is correct. Others have had it working with this configuration. Maybe if you provide some more details behind the issue and the nature of the crashes it can be looked into further.