The current Event-related Triggers and Actions available from the Office 365 Outlook connector lack support for "Resource" attendees. That is, when Resource mailboxes (Meeting rooms primarily) are invited to meetings from conventional channels, the Graph API representation shows them as "Resource" typed rather than "Required" or "Optional". This means they are not available in the O365 Outlook V1/2/3 JSON payloads (and there is no Resource specific entry box in the create/update Event actions). This causes considerable increases in complexity when attempting to manage/track calendar Events with Resource mailboxes as attendees.
A specific example where this greatly changed how my Flow was built:
An Event is booked well in advance in order to hold the Resource (Meeting room)
A copy of that Event's information is kept in SharePoint for visibility and extended functionality (however the Resource is not available via the payloads for capturing/updating)
At a fixed date before the Event is to occur, the Event is checked again (the Resource is still invisible to Flow) and Updated to include a list of opt-in Attendees (done this way to avoid repeated invites/updates as users opt-in)
Whether this Event is to be repeatedly updated, or updated only once as above, the meeting room is lost. It is neither a Required nor an Optional attendee and therefore not available for reference.
The Resource mailbox can be successfully added manually as a Required attendee (engaging auto-acceptance mechanisms and changing its Attendee Type in Graph), but getting which Resource is currently on the meeting means querying the Graph API.
Querying the Graph API from Flow has recently been made much easier, but this is an unexpected complicator for covering a scenario as simple as "Don't drop my meeting room if I 'Get' an Event and Update it with the 'Get' payload".
Further, conventional meeting room resources seem to retain bookings when handled this way, but I have not yet tested whether meeting rooms with booking restrictions will behave appropriately with the Attendee Type change.