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.
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.