If you process a full frame at a time, you lose mid-frame changes to PPU ports and VROM banks unless you've timestamped all writes, and you can't properly detect sprite 0 collisions that happen below the port write.

If you process a full scanline at a time, you lose mid-scanline changes to PPU ports and VROM banks unless you've timestamped all writes. Most games won't be as sensitive to mid-scanline changes that affect sprite 0.