Having used both SASS and LESS professionally, I'll say that it's easy to end up with an over-engineered mess that's difficult to maintain and reason about.

That said, the tooling around both preprocessors is great, so it shouldn't be a huge burden to set it up. Because they're supersets of standard CSS, I don't see a reason why you wouldn't use one. Use the features you like, ditch the ones you don't.