Default Garmin calendar widget shows you all calendars you have access to, whether you've hidden them on your phone's calendar or not.
My entire company shares their work calendars via Google so we can arrange meetings. I hide these on my phone though, so I don't get alerted for Mary's lunch in Singapore with Fred when this event has nothing to do with me. I also use iCal calendars with my family.
The Garmin widget shows me not only Mary's lunch in Singapore, but every other coworker's calendar, and all Facebook birthdays as well. ALL OF THEM. :D So you can imagine how much of my own actual appointments stay visible on the watch.
Garmin should absolutely fix this, since filtering comes standard on pretty much every other smartwatch, but based on my chats with customer support I haven't been getting the impression they're in much of a hurry to do so. Is this something a dev could make? Is the data accessible? I notice someone made a Google Calendar widget, but apparently it's been buggy?