I have a synology nas and is unable to stop the spinning disks. A shame, every other thing in it is nice, but could have beter optimizations, as the disk are the most of its energy consumption.
and I use it like this also for 10 years now. It works great. Only consumes ~4W in deep sleep/system hibernation (the wording depends on from which year you find text mentioning the above nas. Synology decided to reword the function back in some year), and automatically wakes up whenever it is needed. (also not this is not only HDD sleep, it is full system sleep)