The Raspberry Pi is a very tiny, very cheap ARM-based computer that normally runs Linux. The Raspberry Pi 4 is just a bare credit-card-sized circuit board to which you can attach, at minimum, Ethernet, USB drive, USB-C power, and a Micro SD card containing its operating system. If you wish, you may also attach a keyboard and mouse via USB, and a screen via Micro HDMI. The Raspberry Pi 400 provides a housing with an integrated keyboard, making it larger but easier to use when directly connecting it to a monitor. Neither includes a USB-C power supply, so make sure you get one, or a kit that includes one. Or you can repurpose a power adapter from a Mac or iPad. For the Raspberry Pi 4 only, there are also various integrated UPS (uninterruptible power supply) kits you can purchase to protect your Time Machine backup from corruption due to sudden power loss.
Some of the NAS products are a bit better in that regard, particularly if you put an SSD inside them. But, when restoring data, direct attachment to a Mac will always yield the quickest operation, even for a hard drive. For this reason, I focused on network backup to external drives, which can be quickly detached and plugged into a Mac, if restoring via the network is not working well.
Attaching a network Time Machine drive directly to a Mac does work, but it has always been unsupported, and it has its quirks, as detailed below. The volume contains a sparse bundle disk image, which itself contains an APFS-formatted volume with nothing but backup snapshots, assuming the backup was started while using macOS 11 Big Sur or later. Time Machine disk images created with macOS 10.15 Catalina or earlier remain formatted as HFS+J, even when used with newer versions of macOS; these utilize directory hard links instead of file system snapshots, so some of the comments below may not apply. You must mount the disk image prior to restoration.
I have lost too many files during my computing life, some of them valuable projects that cannot be reconstructed. I have always tried to maintain a stringent backup regime, but sometimes events overtake you, like the time when all external drives started failing catastrophically in very quick succession at a time of zero available funds.
Unless you have an old Apple Time Capsule or connect the drive directly to your Mac, the Time Machine feature over a network is basically unreliable. I never could find anything that would reliably over a network with the same features (backup verify and disk repair), so I bought a few extra Time Capsules to keep in service. Time Capsules just worked. There are of course other solutions but none are as good as Time Machine.
As I have mentioned a few times on other threads, a big plus for Time Machine is its ability to alternate backups between several drives. It even reminds me when I forget to connect an off-site backup drive from time to time. I am disappointed that Apple dropped the Time Capsule but other drives are working well for me (touch wood!)
I do get occasional errors when the backup fails to complete, but these are transient, and subsequent backups are ok. Why yes, Adam, I do verify my backups by restoring something on International Verify Your Backups Day as you recommend, and other times. Networked TM is a lot less painful than when I kept a drive connected to the Mac, and was continually waiting with the Finder frozen for 5 seconds when the drive had to spin up.
So if you plan to use one TM disk forever, where old backups will eventually get purged to make room for new backups (i.e. bits get overwritten frequently), this could eventually indeed become a problem. However, if you follow the old adage of filling the drive up only until it has reached about 80% of its capacity and then you swap it for a fresh drive, there should be absolutely no problem with SSD or HDD. The difference is really just cost/GB, noise, and power.
I have a TM drive permanently connected to my OWC T3 Dock and M1 MacBook Pro to which I make regular manual backups during the day (such as during meals) as well as clones of the Data volume to an alternating pair of external drives with the excellent ChronoSync. I also make an additional TM backup to a drive stored in a fire safe once a week.
Instead of backing up everything, I select top and 2nd-level folders from the MacBook Pro internal store and from other attached drives and setup separate backup plans in Arq to save the data to Dropbox. This has been working really well and the only additional cost was that of Arq, which is modest.
This leaves for me, unfortunately, open the question of only powering the backup drives when and if a backup is forthcoming or in progress. Why ? To save energy and in view of my old laCie Big being noisy event when its drives are unmounted. 2b1af7f3a8