Honest question, what is defining "open" here?
There is a difference between "open" and "depth/breadth of 3rd party product support".
ACC is *very* open in that there is a robust SDK/API (yes, I know you don't like hearing that, but wait...) that almost anyone can leverage to add functional integrations to the platform. Camera support is *mostly* done via ONVIF. While not a perfect example, we also at the same time hear many people claim that the industry needs to do more to adhere to standards instead of custom 1-offs. Avigilon uses ONVIF heavily in their own cameras, though I'm sure some things (like pixel-motion) are not ONVIF yet.
I bring up the SDK, and really this is essentially the same version of "open" as Milestone (the primary example being used here). At VideoIQ when we wanted to integrate our edge storage, analytics alerts, and customizable meta-data overlays on video it became clear that "open" was kind of a misused term. There was no easy or direct path to get full support for these features in Milestone (or several other "open" VMS platforms).
If Milestone is so "open" (and I don't mean to pick on them specifically), then show me how you can implement your own custom video codec into Milestone easily. Show me how you can integrate your own edge-store API/format into Milestone easily. Show me how you can add customizable meta-data overlays to a video stream easily.
Open, for essentially all the VMS's on the market, tends to mean "we have some formats for you to interface to our primary functions. you most likely have to do all the work yourself, and sometimes get it certified, and then scheduled for release in a driver pack/update"
I don't think there is any VMS that is open to the extent any random hardware startup can create entirely new functionality or innovation and leverage it directly into the VMS.