What differences are there now adays from Chrome OS and the Android OS?
Android on Chrome OS (ARC++) is just one supported container flavor sitting on top of the Chromium/Chrome OS kernel. Not all features of Android are supported, the version of Android (Nougat) that Chrome OS is running depends upon the version of Chrome OS.
See: Running Custom Containers Under Chrome OS for more details regarding containers on Chrome OS.
So while Chrome OS has a high security reputation, getting Android to run on a Chromebook means some compromises:

Don't Android apps (ARC++) run in a container and not a VM?
Unfortunately, yes, Android apps currently run only in a container. We try to isolate them quite a bit (using namespaces, seccomp, alt syscall, SELinux, etc...), but at the end of the day, they have direct access to many syscalls and kernel interfaces, so a bug in there is reachable via code compiled with Android's NDK.

Additionally while Google is trying to promote Android on Chrome OS to developers: https://developer.android.com/chrome-os/ there are still outstanding issues like getting Android on Chrome OS to see the SD card in your Chromebook which might be considered showstoppers by certain apps.