What are the pros / cons of a setup like that (i.e. server chasis with lots of 5.25" bays + CSE-M35T) compared to a chasis with integrated hot swap bays? I can see how it helps with repairs (e.g. if a drive bay / backplane fails, then there's just one CSE-M35T to replace). Are there others? I was thinking of buying something like this:
4U Server Case w/ 16 x 3.5" Hot-Swappable SATA/SAS Drive Bays, 12Gb/s HD MiniSAS + 2 x 5.25" Bays ... but now I'm wondering whether there's a better solution for my needs (want to run a NAS + a few VMs, including a Windows VM with an optical drive attached)
The problem with stuffing a case full of these drive cages is that it costs far more than just buying a used server chassis. The cheapest I've seen for these is $130 used for an M35. That puts you at $390 for 15 drives. Meanwhile if you wait for a decent deal you can get an entire used 847 for that price (36 bays).
Then, there's management. If a drive needs to be replaced, you don't want to be guessing which drive bay it is and risk pulling the wrong drive. Nor can you just inspect drive serial numbers like you can with loose drives in a PC case. Thus, you need management. The M35 "B" models support sideband management, but the problem is that each sideband signal is intended to manage four drives. Thus, you need two sidebands (typically that would mean two HBA ports) to manage five drives. This means you'd need more HBAs or expander ports. Meanwhile, if you use a backplane with a multiple of four drives, you avoid this issue. Or, if you use a backplane with a built-in SAS expander, then the backplane itself handles the management. The one you linked seems to be unmanaged (or at least I can't find any information about it).
As for replacing single backplanes, backplanes don't tend to fail that often, especially if they don't have an integrated fan. Server backplanes on the secondhand market really aren't that expensive - a 12-port SAS3 backplane with 4 slots that also support NVMe is down to $120.
I've spent a decent amount of money on these mobile racks/drive cages over the years, and it almost never ends up being the best option in hindsight.
Now, the requirement of an optical drive is somewhat of an issue, but if a slim optical drive would work (or even a USB), that opens more possibilities.
Also, worth nothing that a solid server chassis is an investment - pay a little more up front, but you'll be able to re-use it for many builds to come, and you won't get nickle-and-dimed over things like drive cages.