[BUG] FFmpeg 'color adjustment' messes up 4:2:2 colors

[Selur]

Can I ask why correcting basic brightness/contrast with FFmpeg/Mencoder is not recommended? Is it really bad, or is it just not as good as Avisynth/Vapoursynth?

Simply not as versatile and good as Vapoursynth/Avisynth.

I use it because it so much faster than the latter: generating the preview files from lossless FFV1 videos (from an external harddrive) takes a lot of time, while FFmpeg filtering has instant preview.

That depends on the source filter that is used.
FFVideoSource will create an index on the hdd which is slower than, LWLibavSource which creates the index in memory.
Indexing is important for most filters since it allow accurate access to frames.
Also note that Hybrid only creates the index during the time opening the file, later it will reuse the index.

Cu Selur