XP or not : no utter software development methodology

I've practiced XP for about four years, and I believe it's a excellent methodology for developing robust software, but recently I doubt what I got from XP, it become even worse when I read "Code Complete 2nd Edtion"(CC2E) those days

- XP said unit testing is very very very important, so we should testing first, CC2E said test is important but not so much

- XP said write code immediately, and improve software during iteration, but CC2E said if you don't get a clear/detail requirements before starting, you will be a loser, and improve software during iteration isn't a good way to follow, if we can do it well at starting, then it's the best way

- From my practice, I met delay delivery several times even we use XP, and also a bad stable status of our software, I know it's not all fault of XP, but from CC2E I do know XP isn't a silver bullet too, XP isn't suitable for every type of software development, it has its own suitable area