Earlier quoted context omitted.
Two words: URL shorteners. That still introduces the problem of surprise, but with the space saved by not posting the QR code, you can provide context about what the URL does provide. And riddle me: why can't your phone to OCR on a legibly-presented URL any better than it can read a QR code (yes, I know, redundancy and all that, but still).
You've said it yourself - with a shortener I've got no idea where it's going, and now I also have to type a weird, meaning-free string of upper, lower and digits. And I don't have a reliable OCR app, and OCR doesn't have redundancy. So it doesn't work, it's hard to type, and I don't know where it takes me. How is that better? And at least scanning the QR code shows me the URL before opening it.
I suppose, client side, you've got the option of previewing the destination URL before going to it either way. Again, not that a naked URL tells you a whole lot, but at least I can see if it's a PDF, HTML doc, or EXE downloadable off the top.