AVBlocks does not process video streams with Variable Frame Rate (VFR) reliably. This will affect H.264 and VP8 streams which are encoded with Variable Frame Rate. However, most H.264 and VP8 streams are only marked as Variable Frame Rate (VFR), but are actually Constant Frame Rate (CFR). Such streams will be processed correctly by AVBlocks.
Internal video transformations are always performed in 4:2:0 chroma subsampling mode. As a result, processing video input with 4:2:2 or 4:4:4 chroma subsampling will lead to slight loss of video quality.