There are flavors of android including vendor modified or changed specifications. They can be subtle, like a pay as you go phone carrier, or major like what Amazon has done with Fire. Each of these changes introduces security issues outside of Google's control.

With a Chromebook you get the ChromeOS from Google regardless of the vendor who made the hardware.

At least that is my understanding.

Well they'd only be fine if it were equally reliable and consistent, which is not a sure bet.