HubConnect has changed significantly in it's year of availability. @srwhite is constantly tinkering and has new ideas every 2-3 hours. Therefore, some of the most useful messages in the Thread are from mid-November to today... Those deal with the specific changes that got us to where we are today and v1.6. Therefore, I'd suggest reading the first 5-10 messages of the thread, then skip to mid-november and read to the end.
Take a look at the Videos and Installation instructions at:
Not automatically, you must identify which devices are to be shared.
There are two methods available to interconnect hubs: http (oAuth) and Event Socket.
http (oAuth) is ideal for interconnecting hubs on different subnets.. or over the internet. There's two simple examples: SmartThings, which only has a cloud interface. Messages to/from SmartThings must use their Cloud and http is the only option. Second, hubs that are physically distant. If you use @srwhite as an example, his RV would be exactly why the feature exists. The only way that 'mobile' address space is available is via Hubitat's cloud and http.
EventSocket is using Hubitat's (not publicly) supported service that is intended for Dashboard. It's not so much 'at risk of going away' as it is of being altered, to add new features to Dashboard. Hubitat certainly knows of it's use for several projects and won't remove it without a) a replacement and b) an announcement. The BENEFIT of EventSocket is that there's no 'per-event subscription' that consumes resources.. especially when doubled or tripled. Every event gets sent to EventSocket (if only) for Dashboard. It's unfiltered and from a processing / resource consumption viewpoint, free, it's 'always on'.
HubConnnct listens to the Event Socket stream and then must filter and de-duplicate. The presumption is that the time critical tasks of automation would run on 'this hub' and the tasks of distributing the events would fall on 'that hub' -- a hub with far less to do and thus able to filter.
This model is certainly working well for me, and by the 'sounds of silence' in this forum, the vast majority of homes using HubConnect. The result is the same though... the difference is in the minute details of where the filtering takes place. For my system of 3 Hubitat Hubs + SmartThings + Homebridge, both methods are used. EventSocket for everything but SmartThings, which must use http (oAuth). I have two more Hubitat hubs for development and they change how they are interconnected every few hours as I look at this or that.
The shorter version of the above would be: Small numbers of interconnected devices can use http (oAuth). Above a certain point, and I'd say 50-60 is a good evaluation number, Event Socket helps a little bit more. For extremely large systems, over 400 devices shared, is where some additional features of HubConnect can really help. (I'll leave the tease to @srwhite )