Hi - in my theme the button placeholder text is unreadable. This will happen on any theme that doesn't have dark coloured buttons. The reason is this very specific rule:

https://github.com/WordPress/gutenberg/blob/master/packages/block-library/src/button/editor.scss#L17-L19

You can see a screenshot of the problem here:

and here:

If you change the colour from white to use the currentColor keyword then the element will inherit the buttons colour and so always be readable. The 0.8 opacity will help show it's a placeholder and not normal colour.

I have used this fix on the editor styles for the two themes screenshotted above and it seems to work fine.