For UI scaling, it should definitely be enough to just rely on DisplayMetrics. Do you really need a "sanity check"? (and how would you do it? A big list of device IDs? All you have is the information you get from DisplayMetrics, afaik there is no other way to know the physical size of the device) I still doubt there are devices which report truly garbage values there. If they did, wouldn't all apps fail on that device, since the application framework relies on DisplayMetrics internally to select resources for and scale the android UI?
If a device manufacturer have chosen to report a very different DPI (like 180 instead of 240) it would mean all UI on that device looks smaller then on a "normal" android device, but in that case the user is likely to be accustomed to a smallish UI.
It's of course a nice service to your user to provide a UI scaling option, but I don't think it is something that need to be a priority.
I totally agree that it would be really nice if the device manufacturers got their shit together and just reported the actual DPI though... In our game we have a multitouch gesture engine with lots of complicated and quick gestures, and if we don't know the physical size of the device we can't give a consistent game play experience across devices. There are other issues that makes that practically impossible though, so we decided relying on DisplayMetrics is "good enough".