Hello Jukka.
What would I lose if I used NetBSD as xen dom0?
Theoretically you would only loose sparse file capability. And if it was a XEN cluster then you would also need to consider some network storage specifics.
In practice you first need to check that all your peripherals are recognized by NetBSD and that disk performance is alright (assuming this is hardware raid). Then you might loose some flexibility. I doubt one can simply build latest XEN releases and make it happen, while that works pretty well on BSD-like system like Slackware Linux *1.
*1 https://slackbuilds.org/repository/14.2/system/xen/