People have fussed the same of the C preprocessor, around the same time I and maybe you were born. (There's a pretty good chance I'm your parents' age, and nearly no chance you're the age of mine.)
The criticisms were valid then, too. C (including the preprocessor of course) is still not fully parseable if you include things like token concatenation.
I make no representation as to soundness, then or now. Not till I figure out where my copy of the UNIX-HATERS Handbook has got to, at any rate. I've had cause reasonably recently to reread the X and sendmail chapters, not so much this one.
The mistakes embodied in both thus far look not just still relevant but positively timeless. Certainly, to judge by how often young people with no sense of their field's history recapitulate those mistakes.
Without specifics it's difficult to evaluate what you're saying.
What are some examples of the timeless mistakes in those programs? I think X was a pretty good effort, it's just that it essentially ossified and has been left behind compared to some more modern systems (although I'm using it right now.) Sendmail approach to dynamic configuration was sub-optimal. But these aren't examples of mistakes that I see recapitulated often.
Not every configuration system is as bad as the m4 nightmare that sendmail used, and I understand nothing really better was feasible in the prelapsarian or Stone Age days of its implementation. But I worked - fought - with sendmail for years and, as in the book, I also remain mildly surprised that Allman continues to perambulate. Most such things in my later professional experience differ by degree, not kind. YAML is not as bad as what was typically perpetrated in Perl days, but it does too much and too little and all its fiddly rules give me headaches. JSON is awful and what we're basically stuck with, because even though it's so simple it's almost useless, at least it's simple. XML is much better than it gets credit for, but nobody likes it because most programmers seem to regard the need to use a keyboard as an imposition, and I assume also have frequent nightmares featuring lots of pointy angle brackets. (I use Emacs because I don't hate myself, and I wish more people had the sense to keep things as simple as Emacs Lisp typically is.)
I don't want to talk about X. Wayland has been about 60% mistakes by volume, and I like too many of the people who made it too well to be anything other than sad about that.