I have results from 3 different post motion blur effects.
1. Furnace motionBlur in Nuke with no motion vector.
2. Reel Smart Motion Blur using la maison 2d motion vector shader.
3. Nuke's built-in vector Blur effect using Maya's 2d motion vector render pass (normalized one).
Here's the comparison in color channels.
Apparently, they all look very similar and showing a reasonable quality.
But in alpha channel, there are some differences.
The first one, Furnace motionBlur shows some artifacts. For example, you can find some smudging around the ball especially when the ball is being released from the hand.
This not-so-clean alpha is not desirable for compositing. Since the Furnace motionBlur was not helped by any motion vector source, this kind of artifact should be unavoidable.
The other two effects are done with a proper motion vector source provided and did generate a relatively clean and smooth alpha.
RSMB still produced a better alpha than Nuke's vectorBlur.
While the ball is exiting the frame, the edge of the ball looks better in RSMB's case ( the center )than Nuke's vectorBlur ( the right ).
Overall, the winner is... Reel Smart Motion Blur (imho).