15 million lines of code - that is a burden guranteed. But then also when you look at the way the kernel is designed it is easy to debug the kernel.
Also you are forgetting the fact there are possibly thousands of developers working on the kernel. Ofcourse, there is this danger of a bug creeping in - that is the reason why stable distros (slackware, debian, redhat etc) use only the tested and known good kernels.
So unless you go out of the way and install a kernel that is a beta or a release candidate I am pretty sure you are good to go