Apple do advise even public beta users to not use the beta on critical devices plus to always back up. Often, when troubleshooting an issue, they will also ask users to restore the device to see if it clears the bug. So it comes back to the fact that beta testers are not all actually using the devices in the same way as average users on the public release - meaning bugs are missed. Developer betas are always strictly not for use on a primary device.
There is also often a problem of “software matching” regarding iCloud features. By this, what I mean is that the new software can change how data is handled in iCloud, breaking compatibility with old software. It’s all fine if everything is on the latest releases, but it can cause issues if you are running a mix of software. This year, for example, this has happened with upgrades to the Maps app. There is all sorts of stuff about this in the beta release notes.
The trouble is Apple have now released iOS 14 without releasing macOS Big Sur at the same time, so this sort of issue is likely to crop up. Ideally, they should release all their new software versions on the same day (or if you have lots of Apple devices and features like Continuity and Handoff are very important to you, it’s best to wait until you can upgrade them all after all the new software has been released). I think Apple takes a relaxed view about this as they know most users probably have only iOS devices and aren’t affected, and some will have Macs but not care, while still others will be technologically-minded enough to know not to upgrade yet.
As others have said before me, it all goes back to iOS development having to be set not to a software timetable but a hardware one - meaning that Apple often has no choice but to ship software even if it is not ready in order to meet deadlines for hardware releases. They consider this to again not be a major issue as, with almost everyone having access to the internet, they can always release patches later. But it does mean you have to expect the initial .0 release to be a bit rough. Last year, for example, Apple knew iOS 13 wasn’t ready but it was missing the shipping deadline - so they packaged up the beta code into a GM and started loading it on iPhone 11s at the factory. At the same time, they incremented the build numbers to the .1 branch and continued beta testing (now on a release which was nominally iOS 13.1 but was really always intended to be considered as iOS 13). They never meant for anybody to actually use the iOS 13.0 build, but it was something they had to put on iPhones at the factory and then release to the public so they could back up their old phone on iOS 13 and transfer across to their new one easily. If you weren’t upgrading to a new iPhone, you were really supposed to wait until iOS 13.1. An interview with an Apple engineer later revealed that they only expected these people and “early adopters” to upgrade straight away to iOS 13.0.
iOS 14 is supposed to be something of a less-major release, and it isn’t too bad now, but the last real Snow Leaopard style release on iOS was iOS 12. Bug Sur, though, is a major update this year and probably why it hasn’t been released yet as it isn’t ready.
I personally run the betas on my main devices even though Apple advises against it, but I make sure to do both the macOS and iOS betas to keep my Mac and iPhone in sync. I have seen no issues with iCloud features this year as a result. Since Big Sur isn’t being released yet, I specifically told my sister yesterday to switch off auto-updates so that her phone doesn’t break compatibility with her Mac.