When I get SketchUp 2021 files from people that show issues, or even fail to open, I send those to the developers for them to see if there was an error that the validity test should have picked up on. Fortunately there have only been a couple of those since November, and one led to an improvement that already went into the 2021.0.1 update.
A more recent example is this one:
Naming materials, styles, scenes, or watermarks, with more than 260 characters, leads to an issue in the 2021 file format. For the problem case two of the materials had very long names. Instead of just fixing those two materials, the developers could see the underlying issue, and will fix it for all four cases.
So, if you do see something where using the same file and doing the same action would have left it in a different state to how 2020 would have left it, or if it leaves the files in a state where they won’t open, please let me know.