Any special reason (apart from its huge number of bugs and amazingly poor integration with partner projects and plugins)?

Not needed in ~90% of use cases. Clients too dumb to use it without routinely screwing it up, so better off with plain vanilla HTML5, and get edits done faster, better, cheaper by maintenance contract.

Oh right - you mean as a supplier where the client specifies it. Yes, I can see that would be annoying. 

Apart from that, I just find it super-slow and kludgy to populate with content through the CMS. Minor edits are ok. The template structure is also very constraining, and the visual designers here aren't very mindful, or appreciative of constraints. Custom code nearly always works out better.

What you're saying is that as a website supplier you're more comfortable with the CMS you've written yourself, which is like duh, obvs!

That's fine too - I use lightweight or self-written frameworks for my own sites that only I deal with, but when I've had a site to hand-off to someone else, using Wordpress means I don't have to explain what I did, and I've found it to have different shitness to the other CMSs I've used. (If there's a good CMS out there, I've not used it.)

Which is not to say there isn't plenty of irritations with WP - I remember menus being a particular pain, and there was a fucking tonne of terrible plugins to navigate through to find a workable gallery, and it could and should be WAY more secure by default.

But there's nothing wrong with software projects focusing on where their skills lie, and using Wordpress (or Drupal/Django/etc) to provide their site.

All of which is nothing to do with the originally asked question:

   What do people think about having a federated libre communication platform (like Matrix/Kune/other) for Teh?

Sorry, I don't like any CMS. I don't care if some random software project chooses to represent themselves through a CMS (though I consider it inauspicious). I just.don't.like.them.