I think there is ample evidence and no doubt that power cycling hard drives, particularly older hard drives, will shorten their life, and they typically fail when rebooting a PC. Newer drive designs are better, but they will still fail.
But for a home computer, unless you have a very large UPS and/or solar power with battery or alternate backup power supply, what will happen when you get a power failure if you have been running a very old drive that is way past reliable restarting age?
You are going to get a failed drive straight after the power failure.
We are talking about Windows PCs here. They occasionally need to be rebooted for Windows updates, depending on what you were doing at the update time and the type of update. They occasionally just need a reboot because some software mucked up, and memory usage is a mess. Or a driver locks up a process or something, so a reboot is the only way to get the PC working again.
While a home user running an OS like Windows may not be able to avoid having to ever reboot the system, you can still minimize the number of power cycles that a drive goes through by keeping it running 24/7 and preventing it from sleeping.
You may still have to reboot once or twice a month for updates, but the drives are not going to be power cycled daily or even several times a day.
I'd think that a reboot is at least a
bit less stressful than sleeping or hibernating the system overnight too. At the very least, it's going to remove some amount of thermal cycling since they aren't going to cool down to ambient temperatures. I believe that one of the surprising results from one of Google's studies years ago was that failure rates were higher when drives were cold.
Regarding the UPS: it depends what you consider "very large" but a 900W UPS is smaller than a standard mid-tower PC case - maybe 1/3 or 1/4 the size, and can keep the system running for a few hours.
I have Windows set to immediately do things like drop the CPU clockspeed, PCIe link speeds etc to the minimum as soon as it switches over to battery power to maximize runtime, and if it is going to run empty, it will at least hibernate cleanly rather than cutting off.
I'd rather have a PC that I know will reliably reboot when required, until a disk fails. I cover that issue by running Hard Disk Sentinel, which I'm hoping will predict imminent failure so I can replace a drive before it fails. So far, so good.
You might not want to change something that is working for you, but I've switched from HD Sentinel to StableBit Scanner.
I still use HDS for things like initializing and testing new drives before I put them into production, but StableBit Scanner actively runs tests on the disks it is monitoring rather than just pinging the disk every five minutes and checking for SMART errors. I believe it aims to complete a full surface test of each disk once a month.
If you use StableBit Drive Pool to pool your disks together, it can also start migrating data off at-risk drives immediately, and prevent anything new from being written to it.