While designing for the web or the mobile applications, a natural free flowing text can be used for the general content, but when it comes to actions... can we use CAPITAL LETTERS.
As a general rule of typography using all caps reduces readability for long segments of text. It should never be used for paragraphs especially, and only for short headlines.
You can certainly use all caps in buttons, though, because buttons by nature contain short segments of text. In fact, all caps is the convention for buttons in Google's Material Design. You can see how they specify this at https://www.google.com/design/spec/components/buttons.html#
One thing I notice Google doesn't do is increase the letter-spacing when using all caps. In general it increases readability to add some spacing between all cap letters. See http://practicaltypography.com/all-caps.html
I don't see why not to use capital letters for buttons and actions. It can be quite nice and add visual discernment to your app. It would be wise to watch for device visual standards (e.g. Android Material and iOS have different visual standards).
You should also take care to make enough of white space around text to make it more clear and readable.
This is one example of readable button in web app:
There is nothing to stop you using capital letters (all-caps) to indicate actions. It should be noted, however, that there is no common pattern for all-caps indicating actions.
If you are using the pattern across your system then you will be encouraging your users to understand that, wherever they see all-caps within your system, there is an action available to them.
In short, other than basic readability, there is no real issue with using or not using all-caps to indicate actions.
The only thing I would say in caution is that you may find this limiting when it comes to formatting other text that may require all-caps but is not an action.
How about use Small caps (https://en.wikipedia.org/wiki/Small_caps)? I guess Visual designer needs to look at it to be sure it will look ok. But its nice idea for button label (especially when you using serif fonts).
I really don't recommend you to use capitals for this case. For two reasons : capitals are less readable than smal cast and it's like if you were SPEAKING LOUDLY to your users.
There are many beautiful and efficient ways to highlight your content elements like a call to action without using capitals.