For large/complex sites, you should take into consideration the performance of various selectors. ID selectors are always fastest, and single classes are faster than complex selectors.

Nested rules in LESS/SASS make things easier to manage, but when you end up using extend blocks, nested files, and parent rules, you can end up with very non-performant CSS rules. It may not matter much in a small page, but on larger pages with thousands of DOM nodes it can add up.