My Photo/Audio workstation was a 3,1 and I moved to a 6,1 and I'll tell you this much, you're better off with a 5,1 than a 6,1 just in general. I have phantoms causing reboots that are somehow GPU related (D300's in mine not the ones part of the Apple warranty program for the 500s and 700s) but very difficult to deal with and I think I'm making progress on isolating and mitigating the root cause but my workarounds are not ideal.
Some Thunderbolt 3 docks work fine with the trashcan6,1 but others work only sometimes or occasionally. I used my OWC TB3 dock for like two weeks when my MBP was away but then I hooked it back up and didn't even light it up! so I'm using an old CalDigit break-out box dock to get me 2 eSATA ports that I use because I've got zpool attached.
The internal storage on the 6,1 can be taken to a 2TB NVME with an adaptoid, you can go up to 128GB of RAM. This is the thing that keeps me off the M-series Apple Silicon Macs right now; I'm used to needing 16/32/64/128GB of RAM for my workloads and I don't even do much that demands it, mostly Lightroom, Firefox, Affinity and extremely rarely Photoshop. I would be fine for Lightroom on an m1 with 16GB and maybe even 8GB but I am super skeptical about anything less than 32GB for any computer I buy anymore.
There is a 70% chance that my trashcan6,1 will turn into a macpro7,1 within the next six months and I'm not thrilled about it but my 2008 macpro3,1 was still a daily driver until a year ago and I haven't decided what to do with it yet (that era of Xeon doesn't have instructions docker/VMWare Fusion expect so I can't even use it as a multipurpose hypervisor but apparently vsphere/ESXi will work, I just don't know the impact of those missing instructions in my pair of CPUs.
The 2013 is a workstation that I'm both impressed with and disappointed in.
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for anyone curious my watchdog reboots and GPU crashes/WindowServer eruptions — they're like everyone else's and I've tried cranking fans, disabling stuff. I've got an LG Ultrawide Thunderbolt 2 display. that's right Thunderbolt 2 NOT miniDP! has to be thunderbolt 2 cable or it won't work. last night after the 20th crash last week i am following a hunch and knowing the allocations of lanes and busses for those TB2 ports I've moved my monitor from 3 to 6 and moved my dock from 4 to 5 and i'll ditch TB2 for display and try HDMI from the HDMI output next.
i can't even run with filevault because i travel between two residences 900m apart and last month my trashcan crashed at the login screen and I couldn't export any photos for getting prints made without doing a super tedious song and dance with the backups of my NAS that it replicates my photography work to, it was a huge pain in the ass so I can't even follow my own best practices by using WDE on the boot volume. My zfs datasets work with/without encryption fine, i've got like five SSDs hooked up to it using jhfs+, apfs, zfs, my JBODs of magnetic disks (zpool) but someone can just roll in and yank my 2TB NVME out and they've got a lot of important data that would allow them to access even more important data, so i'm feeling really vulnerable at the moment.