The main point of using a Raspberry Pi is for access to the GPIO pins and the built-in camera interface. As others have said, there are better alternatives if all you need is a server.
But if one's doing it for the fun of it as the original author appear to have done, then I have no moral complains to present. :)
If Linux is a must, I'd argue that building a Desktop is easier, since you can verify which hardware works for Linux. (Pick motherboards that others have already tested).
Something like a Ryzen 5 2500x build (6-core / 12-threads) would be far better than a Rasp. Pi for any home-server use. You got 4-DIMMs for easy 32GB support for tons of virtual servers. You have Hard Drives, SSDs, AVX2 (for high-speed H264 encoding), PCIe (GPU compute) and more. The standard Desktop is a beast these days.
--------
For "clusters", a typical GPU like Vega64 or the RTX 2070 will get you far more performance at far cheaper prices. You can even toss two GPUs on a typical Desktop... maybe four GPUs if you go HEDT (Threadripper or i9-Extreme). That's all a modern supercomputer node is these days: just a normal CPU with 4x GPUs on it.
High-performance SIMD programming is a bit difficult at first, but programming CUDA / OpenCL / AMD HCC is actually straightforward. Its the "high performance" part that gets difficult (but that's true of any programming).
Your "cluster of GPUs" is the architecture of modern supercomputers. You can build one under $1500 (2x GPUs RTX 2060 or Rx 590 + Desktop with a bit more power-supply than usual). That's a Heterogeneous compute with 3-nodes (2x GPUs + 1x CPU) right there, plenty of room for experimentation and learning about communication.
--------
IMO, the Rasp. Pi's main benefit is GPIO and I2C pins, as described by the other poster. Rasp. Pi works extremely well as a host to Arduino (since Arduino has better latency metrics to its GPIO). Effectively, Rasp. Pi is easier to program than Arduino, but Arduino is easier to interface from an electronics point of view (due to lower latency, no OS in the way, etc. etc.)
But from a "serious compute" point of view, like servers or HPC, you probably should just get a normal desktop + graphics card. That's pretty much what a supercomputer is these days anyway.