As this is relevant to the topic: I recently purchased a new TS-264 to replace my failing TS-569 Pro.
After a day of testing, I erased QTS and installed Debian. Debian is far more stable and better supported than QTS. It starts-up and shuts-down faster. It runs far fewer services. Its cron daemon is completely reliable. It’s also what I had been running on my TS-569 Pro for the last several years.
It’s no surprise QNAP Staff would discourage the use of any OS other than QTS, but any NAS owner should be able to install whatever OS they choose.
QTS isn’t maintained indefinitely for all NAS models, so at some point, it’s safer to stop using QTS and start using a supported OS.