Smeared edges, drifting hues, lying scopes
4:2:0 stores luma at full resolution but chroma at one-quarter — meaning a red letter on a white background gets a soft chromatic halo. Skin tones near a coloured wall drift. The scopes in your grading suite read the subsampled image, not the original. Remote tools that downsample to 4:2:0 are fine for screen sharing a Keynote; they are not fine for grading a music video.