Debian testing is way more stable than Ubuntu stable. In Debian's parlance, Testing means "release candidate", actually.
If you want real testing, you should run "unstable". If you're in for fun, try experimental.
Running Testing on my desktops and stable on my servers since forever. None of them broke, none of them created problems.
I don't care Stable running 6.x.x. It gets security updates in less than 24h, and is applied automatically before I wake up, and doesn't ship coreutils with 104CVEs, just because it's Rust.
Testing doesn't get timely security updates. It's not for serious people with production systems. It shouldn't even be a debate.
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Testing is the staging area for the new Stable release.
However, one thing that many people don't know is that Testing enters a package freeze every time a new Stable release is to come out. And this freeze can last anywhere between 4 to 7 months. So for that time period, it stops having frequent updates and becomes essentially a stable distro.