I want to know if WP do any research into how much of an impact the upgrade is likely to have on websites regardless of how good the 3.5 code is
These topics tend to go round and round but I'll try to make my point: this release of WordPress has been vetted, tested, beaten, late-night'ed and tested.
Did I mention it's tested? ;)
Give http://make.wordpress.org/core/ a look and you can get some insight into that process. It's not possible to build a QA test for everyone of the thousands of themes and thousands of plugins but a really good attempt has been made and active plugin/theme developers do get involved and know the score for keeping up to date.
Even with that keeping up to date things happen and many plugin authors have been releasing updates. That's a good thing and that's the way for plugin authors to respond.
give us some slack and make it clear the update may have a good chance to break our websites... even if it is our fault.
That's a common misconception regarding the upgrades. It's never about who's fault something is, it's about preparedness.
If you have a good file and database backup not on your web server then you can turn your installation into a smoking ruin and would still be able to get it all back to the way it was.
That's what I mean by preparedness, being ready to deal with any problems upgrade related or not.
That said, you should do the upgrade and sort out any issues. Normally they're quick to get a handle on but you have to be willing to accept advice from other volunteers and work out those issues yourself.
But you're painting a rosie picture!
Don't worry, you didn't post that. :) But given the tone of these conversations I wanted to get that out of the way.
If you're someone who's got a problem with the upgrade then it really is OMGWTFBBQ?! But from the overall view of the topics and posts the upgrade problems really have been manageable and it's not an upgrade apocalypse. ;)