I like the concept of C4, but when I was using it, I struggled a lot with the feeling that we never really ended up creating diagrams that we'd later use. As in, we'd have a diagram in the documentation, and maybe I'd look at it occasionally if I wanted to remember the name of a particular component, but most of the time when we actually wanted to discuss architectural decisions or explain the system to a newcomer, we would just draw new diagrams on a whiteboard somewhere and discuss those instead.

The C4 model felt very comprehensive, but by being comprehensive, it failed to actually represent the salient points of the systems we were describing. Move up a layer, and you failed to capture some extremely relevant detail, move down a layer, and you had a bunch of extraneous information all over the place. In practice, most of our diagrams would be zoomed in on a handful of components, and represent other components as a kind of convenient lie. Which obviously wouldn't work if you wanted to understand the whole system at once, but that was rarely out goal when using diagrams.

I think it just felt more focused on the formalism rather than the expression, like drawing flowers according to a guide for drawing roses, and then getting into difficulties when you need tulips.