For visual designers talking about accessibility is like talking about death at a birth celebration, which in a way is very sad. At the same time accessibility is seen as a barrier in the path of creativity, which is why we can see many sites with poor contrast and is sometimes hard to read things, but it looks sexy. For me, a good UI must be usable, must offer e pleasant experience, and this is actually the foundation of UX (user experience) but unfortunately today what was called few years ago web designer or front-end designer it is cold UX designer, so actually a visual designer. This is what is expected from a UX designer - push pixels around and inside the screen, paint nicely and that's it.
Read more: UX and accessibility features - the things we tend to forget
As always, each takes whatever technology likes to work with and uses it. Here are my two cents regarding the usability of the three technologies.
First of all, the browsers have a CSS parser / interpreter / renderer, so CSS files can be used straight in any HTML project, which does not apply for Less or Sass. In order to use them in HTML projects you would need to have a compiler which compiles the files (Less or Sass) in a CSS file.
Read more: Less, Sass, CSS - so many options, but when to use what
As always I start with saying that is a decision each should take based on the project and needs. The web is full of disputes and recommendations, based on what is trending or is cooler. Here is how I see it, in plain text.
Continuing on the topic of trends, and when they get dangerous, I will present some previous experiences on the topic. Here you can read the first part.