One of the points is to have a cluster of separate hardware computers communicating over a hardware network. Not for performance reasons, but rather to learn to deal with the limitations.

One NUC is still one NUC. There may even be workloads where 4 CM4 modules (or even Jetsons) beat a modest NUC. Not sure where, though.

NUCs are surprisingly expensive. This plus 4x Pi CM4s is about 300 USD (if your pricing is right - might be a little high). That's still under a NUC.

Ok, new NUCs might be a bit expensive. But I can get lenovo tiny m72 or something like that, with core i3, 4GB of ram and 320gb hdd for <70usd. I would assume 4 of those are going to be similar in terms of size as turingpi build, much more powerfull and consume just a bit of power more (have a few similar machines, they consume something like 10W unless under heavy load)

I felt that expensive is subjective in this context. It is expensive as a full kit NUC, yes. However, barebone NUC is cheaper and I see a few of them are in $300 range with various of i3 and i5. I seen a very cheap $200 range barebone NUC last month, however it use low end cpu, i3 or Celeron.

Yeah nucs really aren't that great. Minimal GPIO if any. No mPCIE slots.... And they are expensive.

The mPCIe connector is mostly dead, it has been largely replaced by M.2 and NUCs have at least one M.2 connector.