I think a major problem with HDMI-CEC from the perspective of folks who want to say control everything with Home Assistant is that it's not "make all the devices on the bus controllable by my home automation setup" but "allow each device on the bus to talk to each other". With no particular standardization about what one device does in response to another or how that's configured, and possibly the vague expectation that the TV drives everything.

As a tiny example, by default when I turn off my AV receiver, my TV notices and turns itself off too. (Or maybe it's the reverse, or both. Don't remember.) This behavior is controlled by a poorly-named item deep in the TV's menu, and by enabling/disabling CEC on the receiver. (The latter is what I chose because it still passes CEC signals through source<->TV and has a better-defined automation protocol of its own.) Who knows if some other TV has the same default or has any way to control it, much less a way I can describe without getting into the specifics of each model.

To make it really useful for a well-controlled HA setup, I think at least I'd have to have some sort of device has a bunch of HDMI in and out ports and intercepts all the HDMI-CEC signals so the devices no longer talk with each other but instead with something I control. That takes it from an O(devices^2) interop problem to an O(devices) problem. Even then I'm not sure it's really tractable. There needs to be more standardization.