What is a HU? A computer that runs an OS and some frontend software usually on an ARM processor. What is a general CarPC around here? Well we have the majority with x86 compatible processors, some that have IBM PPC processors like on the old macs, and some using ARM processors like on that wall plug. Now there are far more (openly available) programs for x86 than for ARM or PPC really. So by choosing an x86 platform for your system base, you have way more than 700 extra things it can do over the ARM.
All stability really is, is a smart choice of hardware and software that play nicely together. If you do it like a cheap bastard and use an old HD, and a P4 northwood powered off of an inverter, yeah you are going to have problems. The HD will probably die, the processor probably overheat, and the inverter cause noise, fire, or destroy components with its crappy sine wave.
As for sotware, any OS works. I'm sure kev will rave about linux here somewhere () but really if you know what you are doing or know how to ask people that know what they are doing, you can lock down any OS to be stable. Blue screen free since mid 2006 for me, with an unoptimized, unlocked, XP install. Not too difficult.
Anyone that takes 2yrs obviously doesnt have a clue what they are doing, are doing something much more than simple HU functionality, having budget issues, or the project is on the backburner...
The good companies already do. For instance the navigation systems on Lexus (well all toyotas actually) is a small modified linux distribution. You can burn a DVD with a special coding in the first few packets, and it will start the system up in a bootloader mode where you can download the data to the HD. It can be run in a VM on a regular x86 machine as well. It is already here, and is transparent. A CarPC can be just as transparent as long as a dolt is the one installing it.