I'm especially cautious about keyboard apps because they are useful keyloggers. It wouldn't take much for a closed app to secretly collect all text, and text is super cheap to store and transmit. I trust their sandboxing just fine, but sandboxing doesn't solve every problem and shouldn't be used as a crutch or excuse to install software that is likely to be dangerous.

The idea, in my opinion, is to have the sandboxing for apps you generally trust just in case. Not to install bad software like Gboard.