I disagree. Compared with other languages and technologies used for similar purposes, none of JS, CSS or HTML is particularly powerful or efficient, nor do any of them provide particularly good tools for helping new developers get up to speed or co-ordinating large teams working on the same project. I initially wrote a huge set of specific examples to support these claims, but on reflection I'm not sure criticising JS/CSS/HTML in excruciating detail would really advance the discussion here. Suffice it to say that best-in-class programming languages, document preparation software like TeX or high-end DTP applications, and representations like PostScript and PDF, have all been doing everything JS/CSS/HTML can do and much more for a long time.

Facebook, Google and Microsoft all use the web stack to run huge companies.

That's not the whole story, though, is it? For example, both Facebook and Google became successful primarily for the hard problems they've solved on the back end, particularly in terms of scalability. Facebook's main web site is nothing special in general UI terms, and their web-based tools for advertisers are awful. The search engine that took Google into the big leagues was famously extremely simple on the front-end, and while today they also have applications like Google Mail and Docs/Apps/whatever it is called this week in their portfolio, again the front-ends for those are still relatively simple in general UI terms and would probably be unremarkable in any other context.

To broadly paint this web stack that so many people poured countless hours into making, as 'impossible' to use is doing it a great disservice.

I don't see anyone saying it's impossible to use. However, it's probably fair to say that the state of the art in front-end web development is between one and two decades behind the general programming community, and that many relatively inexperienced people working in the web field are slowly but surely learning the same lessons (and repeating the same mistakes, and repeating them again) as the rest of the programming world did before. One of the biggest problems, IMHO, is that quite a few of those people now work for influential businesses like Google and Facebook, which means quite a few even less experienced developers assume they know what they're doing. And that's how you get framework fatigue, left-pad, "living standards", sites and apps written more than a year or two ago no longer working in modern browsers, and other such madness.