While the two have some divergence in what they are intended to solve for, grid-row-gap and grid-column-gap (or grid-gap) are the biggest day-to-day wins with grid. I had a fairly straightforward hack using negative margins and calc() to get spacing with flexbox, but the built-in support for it in grid makes it feel hacky and cumbersome to go back.

Grid's fr (fractional) units also make the old percentage-based format for responsive sizing feel unwieldy in comparison.

They serve different purposes, and you'll often want to use both for different pieces of the layout. Flexbox is for laying things out in one dimension, Grid is more for two-dimensional ones.

This video might help show the difference (and all the Layout Land videos are great if you want to look more into CSS grid): https://www.youtube.com/watch?v=hs3piaN4b5I