Just like security bugs, lengthy errata doesn't mean anything. A popular
  MCU will have bigger errata sheet because it gets more eyes on it.

Yeah, no. From all outward appearances STM stuff is basically rushed to market, fix the bugs later. We're talking basic shit like xyz clock input or watchdog straight up doesn't work. More advanced stuff like one of their USB controllers straight up doesn't enumerate with ARM Macs — still not in the errata or marketing materials BTW although the workaround may end up beating you with some other bugs. Or the one family that they had to completely rework the USB peripheral while subtly changing the part numbers. Or yeah no.

> The spreading out over multiple documents is good organization.

No, it's really not. It's things like reading up on a peripheral in the reference manual and then trying to figure out which pins you can use with it. Some vendors will put that in the section with each peripheral, most will include a table within the RM, and STM splits it up into multiple documents — per variant within a family because the families are often loosely related.

None of this stuff is offered up in printed form, they could at least hyperlink it (whether intra- or inter- document).

It's not that surprising really. You've gotta cut costs somewhere.