Signals are too brittle and complex. That's why you have to read three whole manpages to figure out what happens if a process gets the same signal twice in rapid succession.
They are also un-Unixlike: they are used to communicate three or four different kinds of information, and they do most of them badly.
Exactly. Signal mechanism makes sense to notify synchronous errors that arise from the thread's own execution, like SIGSEGV or SIGILL or SIGFPE.
Most of the rest of the traditional UNIX signals are events that should be communicated asynchronously via file descriptors that a process can poll at its leisure, which would be more UNIX-y.
They are also un-Unixlike: they are used to communicate three or four different kinds of information, and they do most of them badly.