> In most cases, I think it makes more sense to use rems. This preserves the button's proportions, its aesthetics. And it reduces the risk of an overflow, if the button has a particularly long word.
Actually, the most appropriate technique, if you are more concerned with overflow and are ok with resizing the button, is just to set a min-width property and padding property.
This gives a consistent size for short-labelled buttons, and allows longer labels to seamlessly scale while achieving consistent padding.
If you would rather button size never change, then your constraint should take place at the label length, and you should specify a font-size.
In truth, user @jacobp100 is correct; it's best used as meant, which is solely for font-related properties.