Be fair, the kernel is actually alright, it's just everything on top of it that is depressing.
Glibc really does a lot of good work to hide the mess underneath.
The biggest problem with Linux is it doesn't have a coherent design philosophy. So some subsystems are nice and others are horrendous. Knowledge of one subsystem may lead to misleading assumptions about another part of the kernel.
An example is the kernel supposedly doesn't have threads, they are just processes that share address space. But of course other parts do in fact need to understand that there is one coherent bundle of threads that compose this abstract idea of a process. So some places differentiate between thread id and process ids and others mix them. Windows has its inconsistencies, but not with something so fundamental as a process.
To see what I mean, try using epoll to manage a set of network connections. They apparently didn’t consider the case where you have more than one CPU and also want to handle more than one network connection. Also, if you do get it to work without crashing on stale fd’s, you’ll find it bottlenecks on a spin lock.
If you want to save some time and jump to the current state of the art, use DPDK or some other user space network driver + IP stack to completely bypass the kernel. :-(