In my experience as a kernel programmer tracing allocations isn't the hard part, it's keeping the correct view of hardware state and the different type of mappings at play, be it an MMU or IOMMU device mapping, and register state. Use after free bugs and overflows do happen, but there are more and more tools that come out every year that can find these things in C code, some of them are even hardware based. IMO the code quality of the kernel is very high and the defect rate isn't greater than projects I've worked on that use garbage collection.