Design for the person who is not a designer
Most of the material your brand produces will be made by someone who is not a designer, in a hurry, in a tool you did not choose. A sales deck, a job ad, an invoice, a conference slide.
An identity system that only works in expert hands is not a system. It is a style that will decay the moment it leaves the studio.
Rules that answer real questions
Useful guidelines answer the questions people actually have: what do I do when the headline is long, which colour goes on a dark background, what if there is no good photo, how big is too big.
Most guideline documents instead answer questions nobody asked, like the exact clear space around a mark that will never be printed.
Components beat instructions
The most reliable guideline is a component someone can copy. A slide layout, a social template, a set of type styles already built into the tool.
Every rule you can replace with a ready-made object is a rule nobody has to remember, interpret, or get wrong.