Just a question on pi clustering in general. Is there an advantage vs buying a single more powerful computer, or a few powerful machines instead of a dozen+ pis?
The big difference is number of cores you have to work with, and to a lesser extent the amount of RAM. For say 800€ you can get probably a 4 core CPU, maybe 8. I guess 8GB RAM? Or you can get 10 raspberry pi 4s for a total of 40 cores and 40GB RAM.
What makes more sense for you will depend on your workload and scaling requirements.
Nope, you build a RPI cluster for the sake of it.
Isn't this more of question on redundancy, ie: "two is one and one is none"? RPi's aren't technically best known for general purpose usage.