Apple's new Reminders app for iOS 13, iPadOS, and macOS 10.15 Catalina isn't compatible with earlier versions of iOS and macOS. Problem is, the current release version of macOS is the earlier version, so while you can upgrade your iPhones and iPads to use the new Reminders, your Mac will be left out of the equation.
Apple does warn users about this issue, but it's likely that users either will misunderstand the issue or not appreciate the severity of the issue.
The problem is such that Apple has published a support bulletin to cover this issue, and buried in this is an ominous warning that doesn't appear on the iOS warning screen.
"iCloud reminders created on a device that's running earlier software are visible only on other devices running earlier software. When you update that device to the latest software, those new reminders will be lost when you open the Reminders app."
And that's not the only way to lose data.
If you create new iCloud reminders on a secondary device that's running iOS 13 or macOS Catalina, but you haven't yet opened the new Reminders app on that device—for example, if you used Siri to create reminders -- those reminders will be lost when you open the app for the first time. To avoid this, open the Reminders app once before you create any new reminders on an updated device.
Also, if you use the iCloud for Windows app, I've got bad news for you.
"You can't see iCloud reminders in the iCloud for Windows app."
Sync is also broken and will have to wait for macOS 10.15 Catalina to drop, likely October 4.
For something as important as reminders, this seems like a messy, chaotic upgrade process that relies on the end-users to seek out a support bulletin to get find out the full impact of the upgrade. If you're already pushed the button to upgrade Reminders, well, it's too late for you and you have to wait for Catalina to land (or, if you used the iCloud for Windows app, for Apple to update that... if that ever happens). If you've yet to upgrade Reminders, I'd say leave it until the dust settles.