There are ways around it. In case of motion blur, edge has a 'feather' where mask's alpha is a gradient. In severe cases, mask's curve has a lot of control segments with each having a different in and out feather defined.

edit:

example: https://youtu.be/yZyIYUEfT3U?t=71

Also, masks themselves can be motion blurred, and if motion blur approximation is close enough to the footage, then it's good https://www.youtube.com/watch?v=biginQL6NIo

And, what it looks like pulling a matte with state-of-the-art tools https://www.youtube.com/watch?v=8oQqr6Lfmag Still a pain.