My mother purchased a special all-in-one unit (brother MFC-8910DW) for the purpose of printing very large PDF files with mixed legal and letter pages. On windows, Adobe Reader + the printer driver handle selecting which paper tray to pull from depending on the page dimensions in the PDF. But on MacOS and Linux, I had no such luck (using both the built in PDF viewers and Adobe Reader). I am unsure if this is a cups feature that is missing from the driver, or some other limitation. Otherwise, the unit's basic functionality worked fine(though I have the scanner set up so it emails a PDF file using the printer's built in SMTP client). I was really hoping to get my parents on Linux but between this issue and a program that my dad uses that doesn't run on wine, I (for the time being) left them on Windows 7. (I had both of them go out and buy new laptops in the last year or two since both prefer 17" screen sizes, which seem to be going by the wayside and Windows 7, which was getting harder to find computers with win7 oem licenses.)

People do not use 'most printers and scanners'. They want to use that one printer/scanner thing they recently bought, and has only printer drivers available for Linux on the manufacturer's page.

Had a problem with this quite recently, and solved it by buying a new device. And, since, as you said, a lot of them work on Linux with minimal setup needed, I was able to buy it without any issues. But this does not mean that the problem does not exist.

Seconded. I bought an HP printer/scanner only months before moving my parent's desktop to Ubuntu. No frills standard print dialogs and GNOME Simple Scan always work without issue. I'm still surprised as hell (5+ years later) that it works as well as it does, and I didn't do anything to make it happen.