All of the 'discovery' features (protocols) can be selectively disabled, and having discovered devices I intend the HE users to choose the devices they wish to create in Hubitat.
In the reverse sense I intend to let the HE user choose which devices they expose using either the homie or HA protocol , or both. The homie protocol, at least initially, will only be partially supported as it is in a state of flux still. There may (or may not be) a further filtering of devices provided by the other platform for selecting devices to add into their system based on their discovery mechanism.
However only Homey is a real contributor to publishing its devices in a discoverable format (homie) . Although both openHAB and HA promote their own standard neither of them implement it for their own device exposure !! It's a very one sided endorsement to get devices into their own application.
HA has an MQTT feature called statestream that does expose the status of it's devices but does not provide control. I have worked around making this bidirectional using a small automation script installed on HA which currently supports lights and switches. More device type support can be added fairly easily using my approach. There is also eventstream which I will explore later if it more useful but it doesn't accomodate MQTT with the same granularity so I chose statestream.
You will also be able to add specific 'ad hoc' MQTT devices into HE by providing the topic/state information when you create the Virtual MQTT device. This is what most people will use I think so I have to make it as easy to use as possible, while maintaining some flexibility
So I'm hoping this will prove flexible enough for everyone from the HE standpoint. They can pick and choose devices they wish enabled from and to MQTT. If there's other concerns you have let me know because hopefully I can then accommodate the needs of more users early on.
NB The first alpha added all 'discovered' devices to HE, next will allow choosing to add only specific enabled devices. ad hoc device addition was untidy in this version, and likely the subsequent release will focus on this.