Seriously. Imagine the fractal nature of software problems (you do need firmware right?), and add that on top of a layer that has poorly documented errata and non standard peripheral variants. I'm currently making a board to solder onto an existing production run because we finally found the 1 sentence in a 1700page data sheet that explained why many of our MCUs are overheating. Had the datasheet simply labeled VREFP as VDDA, it probably would have triggered a review of the power schematic. Oh, and the CPU overheating was masked by component variation, and CPU binning, which means the overheating wasn't obvious on any of the early prototype boards...

And then, there's the sourcing problem. Components that looked like they were in big supply when the hardware was specced, can end up being in short supply, or worse end of lifed while you're trying to get all the firmware working.

The stories hardware devs have about individual chip datasheets causing hours or weeks of pain...

It's most fun when you can prove the vendor's datasheet is lying about some pin or some function, but they still don't update it after a decade or more. So everyone integrating the chip who hasn't before hits the exact same speed bump!