I'm technical director of an agency that builds websites solely in WordPress, and I can say with feeling that Gutenberg has (so far) been a disaster not just in terms of accessibility but in terms of clarity of the development process. It's been a moving target since day one, with us constantly scrambling to keep up with large changes in each release. We held off for a while, and have only built three sites with it,…
I'm obviously in my own bubble, but I don't understand people complaining about WordPress. WordPress is by far the worst CMS you could possibly use, every other single CMS is better in every capacity. Let's go through why - Uses PHP 5 spaghetti code with a handful of garbage event listeners and output buffering - Has no integration with composer at all - Doesn't have any form of content types without plugins - Has no…
I've been critical about Gutenberg, but I'll respond to this by sticking up for WordPress a bit here.
Others have answered the technical questions, but on your point above, that's not what agencies like mine do. We write custom theme code from scratch (although we do have a base framework that we built as a starting point) for clients, and use WordPress as a base system. It deals very well with those kinds of things as long as you know what you're doing. We spend a lot of time with clients up front to understand their use case, and build something robust but user-friendly. The first site we built, for a cross-UK social enterprise with over 200 services, is still going strong. Just because some people (that is, a lot of people) think you can chuck some free hacky plugins at WordPress and get a website doesn't mean that's all it is. It's evolved to a point (well, had evolved to a point) where it was a great base CMS on top of which you could build quite complex websites.
There's a decent ecosystem of paid plugins, like Gravity Forms and SearchWP. If you know your stuff there are also some great open source ones, like CMB2.
Some other reasons why agencies in particular stick with WordPress:
1. People know it, are familiar with its conventions, and actively request it. It allows organisations who are growing to jump from their little Wordpress site to something more complex without having to relearn a CMS. If you're an organisation that's familiar with a CMS there's a significant time cost involved in your staff learning how to use a new one. That's not a good reason on its own for not picking a different CMS, but it's a barrier when the CMSes you're comparing a much of a muchness.
2. Because it's so ubiquitous, if you want to change agency, that's relatively easy, reduing organisational risk. (Choosing an agency that care about quality, however, is a different matter). If you use something that's not common, you risk having to rebuild a site from scratch because no-one else can take it over.
3. There are excellent WordPress hosting companies out there that take away the legwork of doing things like full page caching, CDNs, git, LetEncrypt etc. etc. We use one that for ~$500 a month offers exactly what one of the hosting companies listed on the CraftCMS site does, but we can have 35 separate sites, whereas the same money would get you just two on the Craft CMS host.
Honestly, I think you can do pretty much anything you want in WordPress and Drupal, and possibly CraftCMS too, although I haven't gone deep into Craft yet. What makes a huge difference is getting an agency that knows their stuff and actually cares about both build quality and high-quality hosting.