Mulitple monitor features are not great across a wide range of open source OSs, even though developers are a large userbase of multiple monitors and open source OSs.
What exactly do you mean by that? I never had any issues with multiple monitor setups (or noticed any lack of features..)
I doubt that feature was approached with a "This better make up for the Ubuntu phone!" mindset.
Some features that may be lacking would be some Ultramon offers, like menu bar buttons to send the window to the another monitor or span the entire set of screens, but now that I run a 3-monitor set up with two in portrait mode, that sort of feature doesn't really strike my fancy. Plus dragging works.