1

Google material guideline use Uppercase in button.

enter image description here

Which button do you think is more readable?

https://material.io/guidelines/components/buttons.html

https://material-components-web.appspot.com/button.html

Yahoo doesn't use UPPERCASE and I find it more readable

enter image description here

2

My experience, all CAPS may look more readable when you are using not more than 2 words for e.g. "CLICK HERE" or "SIGN UP". Though Material Design uses ALL CAPS for links and buttons if it is too long. They might have some thought process behind, but if we speak about user's behavior, All CAPS generally means it is very important!!!

Ref. Article http://uxmovement.com/content/all-caps-hard-for-users-to-read/

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