It all depends on how well-implemented, or not.
For example, Toasttab QR codes on the paper tab work brilliantly. They do not require the restaurant to retool with thousands of dollars of mobile devices for servers. Excellent use of a QR code.
As others have pointed out, getting a pdf that does not format well on your smartphone is a disappointing experience, but that is not the fault of the QR code. The problem is in what comes after.
You could be able to order from your table via smartphone, but usually you can't because the landing page is a dumb pdf of a paper menu, and the QR code does not include information about which table you are at, and the restaurant systems need human intervention to know when to open a new tab.