I worked in a somewhat similar project in 2015, though not as complex as this, to build background videos for DotParty, using UE4 for panoramas and then stitching them. One of the hugest issues we found was that, because of this being a game engine, a lot of things are not deterministic, so if we used multiple cards or computers, particles and other environmental effects would not be in sync, and the stitches were glaring.

Yeah, that’s a good point. And taking out the particles and doing those separately is probably near impossible.

For the non-visible screens it wouldn’t matter that much, but they’d still end up with the moving fulcrum for the main engine.

I believe it's been improved in later versions, as they've focused in these use cases, and there might even be deterministic particles now, but I'm not sure, because I've been out of the VFX market for quite a while.

The part you are missing is the insane complexity involved in keeping perfect frame sync with a low latency across GPUs and machines, especialliy when some final compositing of partial outputs is involved. The stuff sounds simple on paper and it looks like you can just go buy the tech, unpack it and switch it on. The reality is nothing like that. The off the shelf tech is fiddly and barely stable because it is always a low priority feature added with the least possible effort.

If you have a 12 frame delay regardless you have an awful lot of time to get your clocks in sync.

Obviously that tech is not simply unpackable, because they’re on the cutting edge. But that’s also why you could expect some customization.

The overall latency says nothing about the sync precision required. The displays need to be synced and the graphics cards need to have their vsync synchronized between them (usually via dedicated hardware). If the displays are out of sync, you immediately get visible tearing at the seams.

If your parallel renderer divides the image along a grid that does not correspond to display boundaries, you need to gather and composite the partial framebuffers after rendering them. This means that you're now sending frames across the network amd you need to take care that you aren't compositing frames from different timesteps, for example, because the the part of the rendered framebuffer that goes to compositor/display node A arrived in time, but the part going to compositor/display node B somehow didn't.

“They could just...”

Pretty much every time I’ve thought this, it’s turned out I was underestimating the difficulty of doing “just” that.

If it were just that easy, wouldn’t they have done that already?

Who knows? Sometimes people make things a lot more difficult than they have to be.

That isn’t always the case, but asking the question is better than the alternative.