Hi @just2sweet, and welcome!
Generally, I had very few issues with upgrades, despite using Copr a lot. RPMFusion should be even less error-prone. One significant caveat: The one problem you can get is that RPMFusion and/or Copr repos lag behind main Fedora, i.e. when you’re trying to switch to F33 repos they don’t exist yet, causing errors during the update. The remedy for that is to wait a bit .
I get exited about new releases (Waiting!?), and I don’t use any system-critical packages from Copr, so what I typically do is simply disable the repos, upgrade and then reenable. That way, any potential issues are limited to the specific stuff I installed, I know the base system is fine and any potential problems are simple to troubleshoot.
I don’t really use RPMFusion, so I’m not 100% sure, but I’d assume the same procedure works as well. I’m sure others here will know.
For things like kernels etc. it’s of course more problematic. The safest approach there, I think, would be simply to check before you upgrade that F33 versions of those packages are available, and only perform the upgrade once that is the case. That way, I don’t think having them enabled during system upgrades is any more risky/error-prone than using them in the first place.