The tradeoff was the throughput of your compilation was terrible. BeOS wasn't magic, it just prioritized the UI over all else. That's not advanced, it's just one possible choice.
MacOS prior to OS X had the same property: literally nothing else could happen at the same time if the user was moving the mouse, which is why you had to take the ball out of the mouse before burning a CD-R on that operating system.
Oh, sure, it was obviously limiting the other tasks. The point was that this is almost always the right choice for a general purpose operating system: no user wants to have their music skip, UI go unresponsive, file transfers to fail, etc. because the OS devoted resources to a batch process.
You’re only partially correct about classic macOS: you could definitely hang the OS by holding down the mouse button but this wasn’t a problem for playing music, burning CD-Rs, etc. in normal usage unless you had the cheapest of low-end setups because a small buffer would usually suffice. I worked with a bunch of graphic designers back then and they didn’t get coasters at a significant rate or more than their Windows-using counterparts, and they burned a lot of them since our clients couldn’t get large files over a network connection faster than weeks.
You can down play it all you want but it was a really nice OS for its time. It's smooth GUI was very competitive to other clunky windowing systems of the time. The advanced part was threading and smp support were woven into the system api making smp development a first class programming concept. Other operating systems felt like threading was bolted on and clunky. And thanks to the smp support prioritizing the GUI made 100% sense. And I believe there were some soft real time abilities of the scheduler so processes with high priority ran reliably.
MacOS prior to OS X had the same property: literally nothing else could happen at the same time if the user was moving the mouse, which is why you had to take the ball out of the mouse before burning a CD-R on that operating system.