Copy any files you care about off, wipe the disk, and install from scratch. This is also usually worth doing with an OEM machine, to get rid of all the crap and weirdly configured partitions they set them up with.
That's a really strong assertion to toss out without supporting evidence. I've had the opposite experience with Debian/Ubuntu (100s of systems, starting in the late 90s) and OS X (dozens of systems, starting with 10.0) — the process is almost always start the update, reboot, resume work with rare exceptions for something like previously-undetected filesystem corruption.
Even my experience on Windows has not supported that conclusion – you might always want to be ready to reimage the machine but it hasn't been rocky for at least a decade.
The difference might starting and staying clean: avoid OEM Windows installs, apply updates regularly rather than putting it off for years, don't install system-level hacks (Mac users were terrible for awhile about using “haxies” and then complaining about Apple when some random binary monkey-patching broke on the next major release), and be careful about installing software from irresponsible sources.
It transpires that the sudo binary cannot be upgraded when su is active, so the entire upgrade cascade-failed and left me with a broken and unbootable system. Had to start from bare-metal.
I've only used macs for a year and only did 10.5 -> 10.6 which went smooth IIRC. Mac approach of owning the hardware and the OS certainly has it's advantage - being able to clone image to an external HDD from my MB pro and then boot a iMac to the exact same image is a magical experience. Setting up a master image on my laptop and then redeploying to an office of 40 something various mac devices using IP broadcast and doing it in an hour - hearing that apple chime 40 times was really cool :D