The UHHB has very few points that are still valid. It is a humorous piece and was written about 3 decades ago. We shouldn't continue citing it as valid criticism.
I disagree. Yes, nobody uses sendmail anymore, filesystems vastly improved and X isn't a resource hog any more. I've reread it some months ago and the main points still hold.
For example the part about `kill`:
Most operating systems have a “kill” command. So does Unix. In
most operating systems, the kill command kills a process. The Unix
implementation is much more general: the “kill” command sends a
process a message. This illustrates the first Unix design principle:
• Give the user power by making operations fully general.
The kill command is very powerful; it lets you send all sorts of messages to
processes. For example, one message you can send to a process tells it to
kill itself. This message is -9. -9 is, of course, the
largest single-digit message, which illustrates another important
Unix design principle:
• Choose simple names that reflect function.I think the UUHB is mostly worth reading to understand the absolutely dismal state of proprietary Unix in the 90s. It explains why technical reasons contributed to the growth of both Windows NT and GNU.