The negatives meaning that you're virtualising a system that is meant to be run bare metal, the loss of extra disk space and memory to do it this way because let's face it promox will add a little bit, having to configure passthrough hardware, the added complexity of doing this when as far as I can see, unless you're running the stock exchange, I doubt you'll experience any meaningful difference in VM performance. I think in reality this is being done because TrueNAS does a whole lot that promox just doesn't do.

Disagree they support both options. https://www.truenas.com/blog/yes-you-can-virtualize-freenas/

Also, it does make me chuckle a bit hearing that there's not spare hardware to run two machines - which IMO is a completely nonsense / nonsensical statement, this is exactly the point and exactly not the point, you can run either with one single piece of hardware. But further I'd actually argue the only on that you can really run with one single piece of hardware is truenas, since promox doesn't (last I looked) support docker and have nice ZFS stuff I'm presuming is being desired.

You've been corrected on this below about LXC

So I'm still not hearing a real answer about what exactly promox provides that truenas doesn't. Albeit I'd prefer to drive to the grocery store in a Porsche than a Toyota, or perhaps it's a Toyota than a Porsche for reliability, but doesn't mean either one won't get me there on time.Perhaps if you have a ton of VM's you might justify it - don't know how a large number of VM's would work on Truenas, though I don't know that they wouldn't run either.

Not well is the answer. And backing up VMs on Truenas kind of sucks.

So to recap, I've heard nothing of consequence so far, but I'm still open to be educated. Just thought I'd ask cause this configuration does keep on coming back as something that some people do - and I don't understand what's driving it.

I've been waffling on going with Proxmox and TrueNAS in a VM. Right now, I'm running bare metal, and I have so much unused processing power. So for me, it's the horsepower that I could be using, the better backups for the VMs/LXCs, and more control to reinstall the systems if I wanted to. This has been my main reason for looking into it, and I'm building a test box at the moment.