I've certainly come across situations where the code and the comment don't line up, but that seems to arise from laziness more than anything else. If the comment is directly attached to a region of code, most developers I've come across will process both at the same time. The area where I see it fall apart most commonly is with javadoc-style comments. Most IDEs will even flag these issues, but they're often ignored.
A comment on its own isn't inherently harder to maintain any other piece of code or documentation. Sure, it's not zero cost, but I find more often than not they exceed the cost associated with not commenting.
A comment on its own isn't inherently harder to maintain any other piece of code or documentation. Sure, it's not zero cost, but I find more often than not they exceed the cost associated with not commenting.