Big Sur beta on external disk vs internal disk? I've read this question: Remove macOS from a APFS container? and this question: APFS - Is it possible to install 2 macOS versions inside one APFS container?
It seems there is a little bit more mess to using (the admittedly very cool) APFS to install more than one OS.
The way the Apple support document mentions it, it seems so easy, and APFS is very modern and flexible.
However, as you can see in the question linked above, it's more subtle. Removing the second OS can require lots of manual steps to clean up the recovery partition, etc.
So my questions are:

Is this messiness still present in Big Sur/Catalina running side by side? Does using a separate container help this?

If installed on an external disk, does this issue still persist?


In summary, Apple's support of multiple OS side by side in separate partitions seems to forget about the contamination of VM, Preboot and Recovery partitions...
In general, I would recommend installing it on an external SSD disk. First, you won't have to worry about possible disk issues with your regular MacBook. Second, it is easier to erase and manage with Disk Utility.
Just as long as you know how to install correctly, I would go for that option.
Edit: While setting up a separate APFS works well in many ways, such as dynamically adjusting the size, the slight risk is still present in case things go wrong.