From my experience (albeit 20 years ago) Debian testing was basically unusable since it could get into an unworkable state quite easily. You were much better off running unstable instead if you needed the absolutely most recent versions of stuff. It would still break occasionally but would get fixes would be available much sooner.

Backports are quite variable. Distro packaging systems are really awkward for software from language ecosystems with many dependencies. I end up having to install a static build of podman if I want a recent version. A backport basically seems impossible.

I have been running with backports for years and can't remember the last time I had a problem. Perhaps things have matured over the years to explain the different experiences.

Backports don’t break a stable install, they tend to either work or are incompatible due to underlying library mismatches (eg libc upgrade.)

However running a testing install can result in a broken system after an apt upgrade and it can stay broken for some time while fixes work their way in from unstable.