Working with android can be a bit of a pain for sure. I'm only talking about android here since that's all I've tested, so don't apply what I am saying to chromium, Android and Chromium are two seperate projects, by two separate teams. but in the end it's important to realize that google has actually needs for these devices. As long as you work within the bounds of needs it's actually not that hard to actually work with. Android has a LOT of stuff for sure.

Android actually allows you to configure most things (granted the documentation is absolutely horrid, Grep my beloved). It is true that most android phones are running proprietary stuff, but this isn't really any fault of google. Google has gone to fairly great lengths to make AOSP a fairly open ecosystem. Nearly every rom is heavily customized as per customers requirements. AOSP can actually run on most hardware fairly easily. Hell it even works just fine on the vanilla kernel (Waydroid for instance). The Issue is that it's nearly impossible to market consumer devices with only FOSS/AOSP stuff, the margins on phones are actually terrible. The biggest issue is finding a phone that can accommodate the more open stuff, not the issue with google pushing crap. In the end Google is making devices for people who will fight tooth and nail to grab the gun to shoot themselves in the foot. A lot of their motivations are based on that. But doing your own AOSP is still easy enough. Just need hardware for it.