The Broadcom CPUs in the RPis have unpredictable pauses to let the GPU refresh the RAM (really). That makes them unsuitable for hard real-time applications where you must service an interrupt within a certain time, or emit a signal on a strict deadline, etc.
They're just fine for soft real-time applications where some jitter is acceptable, but what counts as "soft" real-time versus "not-at-all" real-time is the subject of some debate.
Not at all. It's just that writing drivers and supporting all the peripherals is a massive undertaking (even Linux is often way behind on support -- chips that came out years ago might have poor or no support for some peripherals and the only support you find is in some vendor fork of an ancient android kernel).
There's also the fact that compared to PCs, ARM boards are kinda special snowflakes. DOS could boot and run on thousands of different machines ("IBM PC compatible") from different manufacturers, in part thanks to BIOS abstracting out some of the core peripherals, in part thanks to (de-facto) standard peripherals. Your custom ARM OS? Well, it won't boot on the next board. UEFI is sort of changing that, but really it pushes the problem to the bootloader (until they start implementing UEFI in firmware).
In a way, I actually prefer not having to rely on BIOS or UEFI, because that protects me from stupid implementations and allows me to customize things. And on the other hand, if you want high performance drivers for modern peripherals, you probably don't want to rely on a firmware abstraction for it (but it'd still be nice to be able to boot and get a shell & some basic I/O going even without hardware specific drivers). But that means you do need drivers in your OS, and as long as chips keep changing as often as they do, it's a never-ending battle to stay on top of driver support.
PCs also gave you a nice escape hatch in that you'd plug in your peripherals to a slot and you could choose parts that you have drivers for. That's not really the case for laptops of course, and ARM SoCs integrate most of the peripherals so if you get a new chip, you get new everything.
