In terms of actual real world experience rather than linking to poorly labelled online documentation I can report that we never discovered any fork() related problems in our code when the CTO insisted on running it on his OS X laptop for several years. It ran slowly, and it missed various minor features (because OS X lacked the relevant system call or whatever and we had no reason to waste time fixing it just for one guy's laptop even if that guy was the one running the show) but it wasn't any less reliable and there was no sign of fork() being "in violation of all unix tradition"
There are other systems with a fork-like call that doesn't do what fork() actually does, including OS9 and, if we are to judge from that link, iOS but they don't claim to be Unix, let alone UNIX® do they?
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