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

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.