Blending paths with brushes and width profiles works wrong.
Create two lines.
Apply art or pattern brushes to both of them.
Apply width profiles to both of them.
Ai will generate steps that have uniform profiles instead of intermediate.
Scatter, calligraphic, and bristle brushes won't let you edit the width profile. I did however test with two calligraphic brush paths with different pressure curves and that was ignored for the intermediary paths. Please test this with bristle and scatter brushes as well.
(this is essentially a reopening of https://illustrator.uservoice.com/forums/601447-illustrator-desktop-bugs/suggestions/41634754-blending-paths-with-width-profiles-works-wrong with a feature that was missed when originally implementing it, as such I just copied and edited Egor's description :) )
abc abc commented
When using the blend function on a path to which a brush is applied in Illustrator, the Stroke Profile option is not applied.
When you use the blend function on a general path, the Stroke Profile option is applied to the blend object even if the color and size are different.
However, if you use the blend function on a brushed path, the Stroke Profile option will not be applied unless the shape properties of the two objects are exactly the same.