The default value of the 'box-sizing' property in CSS is 'content-box.' In this mode, the width and height of an element do not include padding or borders, only the content is considered. This can sometimes lead to unexpected layout behavior, which is why 'border-box' is often preferred.

Excessive nesting in SASS or SCSS can lead to bloated and inefficient CSS files, which are larger in size. This negatively impacts loading times and page performance. It is advisable to avoid deep nesting to keep the stylesheet lean and maintainable.