Profile by slope/vertical offset

I get strange results sometimes when using Profile by slope / variable slope / vertical offset where the result has kinks in it. Infact I avoid vertical offset because I usually end up with this result.

The recent example is linear variable slope which references a centreline and projects onto a channel which gradually moves away from the centreline (i.e. not parallel).

Is there something I'm missing? Best practice? Or are these tools just temperamental. I sometimes get the same result with Project Profile to Element if I choose All points but this is resolved by choosing Vertices instead.