Have you tried both? mpv is able to play high resolution HEVC videos backwards at real time by holding the "previous frame" key. VLC can't reliably jump backwards even at second intervals, forget about reverse playback.
VLC makes a choice not to seek backwards to keyframes, which means you get video corruption.
Seeking is surprisingly difficult. Many container formats don't support it at all, because they don't have indexes, and so it's easy to mess up playback or lose A/V sync by trying it. Constructing the index is about as hard as decoding the entire file too.
libav{format,codec,...} are just libraries for demuxing and decoding video. There is huge variability in how those libraries are used, let alone how the video is displayed (which needs scaling, color space conversions, tonemapping, subtitle rendering, handling playback timing, etc. etc.). mpv also has its own demuxer for matroska files, since libavformat's is very limited [1].
