It's partly a case of people mean different things when they talk about "a grid" and how they would use a grid. Bootstrap popularized one simplified model of layout they called "Grid", and MUI is continuing a version of that. CSS Grid is an attempt to unify that simpler "Grid" with more powerful grid constructs and as a more generalized 2D layout engine. It takes the things learned from Flexbox and Table layout and tries to unify them into a sane 2D layout system that has the power of both and additional tools.
I think you can replace nearly every use of Flexbox (and Tables for layout) with CSS Grid and have overall better dev ergonomics. The `grid-template-areas` tool and named areas can be great dev ergonomics far better than anything that came before it in CSS, especially if you have highly responsive layout needs and/or desire to decouple HTML source order with layout order (for better, cleaner accessibility, for one instance; or for cleaner streaming order on slow pages, as another reason).
It has a learning curve more than the simpler "Bootstrap-era" Grid designs, certainly, but I think it's an easier learning curve than Flexbox (but esp. if you've already learned Flexbox, because they share a number of concepts).
I think you can replace nearly every use of Flexbox (and Tables for layout) with CSS Grid and have overall better dev ergonomics. The `grid-template-areas` tool and named areas can be great dev ergonomics far better than anything that came before it in CSS, especially if you have highly responsive layout needs and/or desire to decouple HTML source order with layout order (for better, cleaner accessibility, for one instance; or for cleaner streaming order on slow pages, as another reason).
It has a learning curve more than the simpler "Bootstrap-era" Grid designs, certainly, but I think it's an easier learning curve than Flexbox (but esp. if you've already learned Flexbox, because they share a number of concepts).