Bootstrap got trashed regularly for being the Times New Roman of web development and any mention of it online would summon a howling pack of CSS purists. However, back in it's heyday, you could join a team and immediately start to make an impact because you knew the CSS class names and what they did. You still had to have someone around who knew CSS extensively to work out some corner cases. However, it definitely let the juniors get up to speed and when they hit one of those corners, provided a great mentoring opportunity to help them grow their knowledge of CSS.