Page "Variable bitrate" Paragraph 14

The disadvantage of single pass ABR encoding ( with or without CVBR ) is the opposite of fixed quantizer VBR — the size of the output is known ahead of time, but the resulting quality is unknown, although still better than CBR.

Specifying a higher average or maximum may simply make the file bigger with no discernible quality effect, and an increased maximum bitrate may introduce stutter when streaming the file.

However, reducing these criteria too low will eventually lead to quite drastic losses in quality.

The effect on video is typically an increased blockiness, because the frames are no longer being fully detailed in their rendering.