I'm not sure if that's a bad choice... BTW, I think STIG still requires it. So, if you want to be compliant with some (US) government requirements, you'd still have to put different bits of your system on different partitions / devices.

Why I'm not sure this is a bad idea: a pathological process that quickly captures the entire allocated disk space will be contained by this layout and will likely not affect the system as a whole. Consider, for example, a process that excessively logs into /var/log: once the filesystem mounted at /var fills up, it might fail / hang, but you will still be able to use stuff from /bin or /sbin. But, if your entire root filesystem fills up, you probably won't be able to run any applications at all.