Having been recently upbraided by the esteemed Dr. Mike for not posting enough in this forum
Now I feel like a heel. Sorry, Dr. Steve.
To address this somewhat:
I had a 3,1 which was in fact a freaking nightmare to do anything with. There's something about the bus structure that makes it difficult to get peripherals working. The 5,1 attracts a lot of attention because you can actually drop a pretty high-powered GPU card into it and not only have it work, but have it run Mojave reliably. I have been looking into adding one of those to my studio if I can do so cheaply enough. (That's primarily because so much of my online streaming music is done on Second Life, which is lovely to look at but very, very badly optimized for graphics hardware, so running it on any computer with on-chip graphics and not much in the way of fan cooling is essentially asking to turn said computer into a boat anchor.)
I agree that veering off into Windows/Linux discussion on a Mac thread is a bad idea (and I say so in the forum rules), but those DIY machines have one advantage that seems obvious to the newcomer but is full of pitfalls to the more experienced user: namely, the basic idea of having a big box with a big power supply that you can stuff a lot of stuff into with few problems. A lot of people have real issues with the idea of a computer as a disposable appliance that needs to be completely recycled and replaced when something (anything!) breaks, and even though it can be hard to find PCI cards for certain things, having six SATA drive bays and a ton of USB 2 ports and dual monitor support right in the box is incredibly tempting.
My personal breaking point for Macs is how they are doing internal storage nowadays. A generation or so before they started putting the T2 encryption chip on every motherboard, thereby guaranteeing that lost data on a bad SSD would stay lost, Apple removed a little-known but incredibly handy item from the main logic board: a tiny hidden port that allowed for a direct connection to the internal SSD with no other hardware in the way. This could be used to pull the SSD's data off if the computer were to be made otherwise inoperable; you could even pull your data from a machine whose logic board was totally kaput. When this went away and T2 came in, the reliance became entirely on cloud and external backup, with the SSD being impossible to change out, upgrade, remove for security purposes, wipe when the Mac dies, or recover data from. Basically an iPad with a keyboard and trackpad.
This makes me itch. So much so that I bought a 2014 Mac mini specifically because that one generation had the ability to put in a SATA hard drive or SSD in addition to the existing SSD PCIe card (which of course was one generation before the commonly used and easily available SSD PCIe cards that fit nearly every other computer out there, and is expensive to replace if you can find one at all). Now, of course, it's acting flaky, apparently because the original PCIe SSD is starting to fail. Of course.
The Mac Pro can be a great solution for people who want to have a big box of STUFF, and how well it works for your applications depends on what you want to do and what you put into it. If I do go ahead and try to get one working with a proper GPU and Mojave, I will write it up here.
By the way, I have yet to see any hard practical use-case evidence that Thunderbolt on a Mac Pro of that vintage is anything but a bogus "upgrade" that's mainly done by hardware hackers to show off that they can do it at all. The internal bus speed of those Mac Pros is only the same speed as Thunderbolt 1, and all you're really doing is figuring out how to add shiny new boxes to a beat up old box, which rarely makes a lot of sense.