Author here. Over the years, Zurb's grid has consistently stayed ahead of Bootstrap's in functionality (ex: Zurb was allowing embeddable grids while Bootstrap was still using fixed width) and also in class clarity: "three large columns" vs "col-lg-3".

Beyond these reasons, though, i wouldn't suggest other developers turn their back on Bootstrap if that is what they are familiar with. Its adoption has reached a point where the resources surrounding it are unmatched.

If you are comfortable with bootstrap, stick with it. If you are like me and have a bit of contrarian in you, give Foundation a try and see if you prefer it.

Something to take note of is that zurb grid foundation has ballooned over the years, it used to be way more minimalistic and more modular than bootstrap. Bootstrap and foundation have stolen things from each other over the years and they are very similar today, but generally foundation styling is more subtle and I think that is what he is getting at. The ability to be unique with less effort than you would need using bootstrap.

Also Foundation has a superior grid layout system last time I checked, but dont quote me, I have not looked into bootstrap 4