I don't see much mention in this discussion of how agile development impacts both the rate of bug creation and bug fixes. I only have my personal experience, but I suspect agile both encourages buggier software and longer-lived bugs than other methods.
The problem I see with it is the time-boxed sprints.Developers are rewarded for completing "stories" on time, not for the quality of that work. If a bug is found, they are rewarded for fixing it on time in another story (but only if it is high enough priority to warrant making it on the sprint board).
My experience is that getting it right the first time takes meaningfully longer than getting it nearly right, and agile rewards getting it nearly right. This is fine in the short term, but eventually the system grows a lot of subtle bugs, which add up to some big bugs that are expensive to fix. These take all the bug fixing time and the subtle bugs languish for attention.