I've only played around with it a little, but really like it. I'll probably use it in future projects.
One disadvantage to utility first is one school of thought thought that CSS class names shouldn't describe the style. For example, don't name a class "blue-button". What if you want the button to be green later? I've heard of this from the early days onward, and it's still perpetuated today. I have never felt comfortable with that idea and have broken it thousands of times for myself, so this isn't much of a disadvantage to me. It might be for some who who strictly follow such rules.