They are nice for keeping tools around on spare SD cards that you might not want to run normally. Like that is a good way to look at Parrot or Kali Linux setups.
Checking out how to build an OS from scratch is also handy. It can be an interesting low risk way to explore building Gentoo, Arch, or Linux From Scratch.
The main appeal IMO, is that you have microcontroller like input, output, and serial communications already setup in the kernel with access in user space. As long as the kernel is supported by the Rπ foundation, (it is proprietary undocumented hardware that only they can support), you are getting the security updates required to keep the thing online automatically and safely. The best stuff to build is unique stuff for you that uses these aspects. Like make a little bathroom clock with a little TFT LCD display that tells you the local weather. Then set up some RSS feeds for local community stuff you do not want on your main mobile device, like maybe local political activity, library and community center events, concerts, clubs, etc.
For server stuff, I would stick with devices with purpose built hardware. Like, a micro SD card is slow and unreliable, and the lack of nvme is bad. In most cases it is cheaper to use other old devices that already have screens unless you want to share a hardware design that is repeatable, you need something secure to keep online, or you need serial or input/output. Those are the main benefits.
The thing is, the Rπ is what it is. It is the path of least resistance. The software support is approachable and great. The price is cheap. However, the non profit thing is a scam. The Rπ foundation is basically an arm of Broadcom. The Rπ is a chip from a set top TV box with 3/4 of the die unused. Broadcom uses excess fab capacity to make the Rπ chips and sell them at materials cost. This is not charity. It is controlling the grass roots market to make competitive scaling business ventures difficult. This is why Rockchip is not crushing them already. The Rockchip RK3588 chip is fully documented and open source. In this space, there is little to no innovation, it is only about price on ancient trailing fab nodes. This is the ladder to climb that leads to Intel, AMD, Samsung, and Qualcomm. The Rπ is the guy kicking anyone that tries to climb. So… use it for what it is good for, but in most cases, other hardware is better, and there is nothing wrong with saying so, or moving past the Rπ. I’m lying next to a RK3566 machine right now, sorting out issues with the ARM version of Fedora Workstation, looking at how to build the native WiFi module for it from source, and maybe try debugging an issue in that module’s code that is causing a memory race condition. Although that last one is past my typical pay grade. - So I’m not all fluff here.
That is just my $0.02.
RCA is nonexistent and just branding on contract manufactured goods. AT&T was enormous when Bell Labs was relevant. Now they are little more than a utility service. You are certainly not going to see something relevant like C or Unix coming from them now. Motorola is a joke of a shell of its former 68k self, and in no position of relevant innovation or competition. Intel could not get anything right. They have no competitive products to speak of, they gutted most of the open source stuff that matters and they will never recover to real relevance again. Time will tell. Computing is changing because we are at the final nodes. This is the most critical moment to be on top because the scaling laws are over. Missing the game now is a flop that will reverberate for a century. People do not understand the enormity of what it means to be at the final nodes of silicon. You will see it as the end of an epic within the coming decade. The world of the 1940s-1960s is returning because silicon is the only civilian industry in all of human history that has grown faster than the largest military budget in the world can afford to fund. There has never been another product that is so expensive to build tooling to then create something that is so cheap to produce. It blocks out anyone that wants to invest to compete. This is why it is so critical to fund and build a competitive final set of nodes. There is nothing left to strive for or undercut. There is no ability to fund the node and scale it to compete. There is no room to catch up. The race is over. To fail now, is catastrophic. The final node is less than 10 years away already. But the thing people do not realize is that the hardware cycle is actually 10 years long from initial idea to first products to market. This means the actual engineers on the bleeding edge are already past the end with nowhere to go. There is no more R&D to do for the last +1 node.
So that is why I say Intel. I won’t buy their stuff now.