I don't think usefullness of syntax highlighting depends on complexity of syntax. It's useful not because it's hard to distinguish between e.g. builtin operator

I don't think usefullness of syntax highlighting depends on complexity of syntax. It's useful not because it's hard to distinguish between e.g. builtin operator vs function call, but for some subconscious visual clues and making wall of text less uniform. For assembly it might be even more useful because otherwise code may look like solid column of text. AFAIK, there's no theory why syntax highlighting is useful. There's movement against it, one of famous proponents against syntax highlighting is Rob Pike, with his Acme editor. However, most people still prefer having highlighting enabled.