The thing that is problematic in Dorico with the collision avoidance in parts is that it bases its decision for spacing between staves based on the default positioning of objects.
As an example, suppose you have a rehearsal marker above a system that collides with a dynamic below another. Dorico's collision avoidance increases the spacing between those two systems so that it won't collide. That part works fine. The problem is that now, to fix that unnecessary extra spacing between systems, what you naturally want to do is move the rehearsal marker perhaps a bit closer to the lower system or the dynamic closer to the upper system (or both). The problem is in this case Dorico does not adjust the spacing between the systems, so now there are two systems further apart for no apparent reason on the page, which looks stupid.
To fix this spacing without turning off collision avoidance, you need to move that system (and, individually, every system below it on that page) a bit higher. This is time consuming. The negative aspect of this (as I discovered) is that any editing whatsoever resets the spacing to default. So you can't even go in and respell a note from C# to Db without losing all of the custom spacing settings, and the part looking stupid again.
If you turn off collision avoidance in parts, then if you make minor edits like note spelling changes you only need to manually adjust the one or two problematic systems, instead of everything below that point on the page. You still do lose custom spacing adjustments obviously, but in this case there are fewer staves that needed their spacing adjusted.
Sibelius's algorithms for this are better at the moment. I'm still glad I moved to Dorico (because I save time in other areas), but I think they really need to fix this.