I did a bunch of research for my PhD thesis on this, and came to the conclusion that "the probability of concurrency errors is proportional to the square of the amount of shared state". Threads share state by default, which brings significant risk.
Also server side engineers aren't necessarily safe; there are a number of C library functions that are not particularly reentrant. This has been known for decades because reentrancy is important in signal handling situations. The safest way of dealing with asynchronous signals, in fact, is to use global flag variables and use the flag as an indication that one should e.g. call waitpid because at least one SIGCHLD has come in since you last went through the main loop. You may deny that this happens particularly often, but that particular hack was an important simplification of my life a few months ago.
Also server side engineers aren't necessarily safe; there are a number of C library functions that are not particularly reentrant. This has been known for decades because reentrancy is important in signal handling situations. The safest way of dealing with asynchronous signals, in fact, is to use global flag variables and use the flag as an indication that one should e.g. call waitpid because at least one SIGCHLD has come in since you last went through the main loop. You may deny that this happens particularly often, but that particular hack was an important simplification of my life a few months ago.