Writing CSS is really simple, we’ve been doing that since many years. But the problem is, in larger projects where the CSS code grows over thousands of lines, whether you’re alone or it is a team maintaining and building on the code, it gets really unwieldy (spaghetti code). Code gets hard to maintain, harder to debug, unoptimized leading to poor performance and highly inefficient. New team members start adding more code (classes, IDs, etc.) and start using them leading to a lot of redundant and unoptimized code here and there. Basically it gets hard, really hard. You end up into too many issues.