With CSS names are global. You have to be careful not to accidentally create a class that conflicts with another one used elsewhere in a different context. You have to worry about whether modifying your class will have downstream effects on elements you weren’t intending to impact. It’s a giant pain. I’ve done it by hand for around 20 years before switching to Tailwind.

Can vanilla CSS be used for a complex app? Yes. But, it takes discipline, and I only have a limited amount of that. I’d rather spend my discipline budget on other things.

That said, nested selectors and CSS variables have gone a long way towards making the vanilla experience much more pleasant. I may have to give it a shot on a side project one of these days.