(haha @hikinghack you know me too well… )
In my experience, reliable calendaring and scheduling have always been a big challenge, open source or not. Can you describe the specific problems you are having?
It’s been a few years since I looked into this, but as I recall the reliability of Nextcloud calendars is partially reliant on how well it is hosted. Typically, professionally hosted Nextcloud instances seem to perform better, such as those by fairkom or Cloud68.
There are also open source server software the focuses exclusively on calendars, with a long list here:
Of those, I know that Baïkal and Radicale are professionally developed and commercially supported with a long history, and good reputation. You can self-host or pay for hosting. It is possible that these solutions might be more reliable than all-in-one groupware like Nextcloud.
That said, there are other open source groupware solutions such as SOGo which might perform better than Nextcloud.
In addition, sometimes the client with which you use to create and send meeting notifications makes a difference. For example, with Nextcloud, you can do so directly when logged in with your web browser, or you could use a third party client like Thunderbird (that’s connected to your Nextcloud instance). As much as I like Thunderbird, I know that sometimes it’s iffy with creating and receiving meeting notifications.
BTW, what I noticed is that closed source calendars are not necessarily more reliable, it’s that they are more internally consistent. What I mean is, if everyone uses Google Calendar, it might be fairly easy to schedule things with each other. However, if a Google Calendar person sends a meeting notification to an Outlook calendar person (or vice versa), sometimes annoying errors happen, too!
Hope some of this helps!