I haven't used d3 but I used chart.js a little while ago. At the time is was relatively simple but lacking some features (like support for interactions). Looks like this newest version has added some of that which is great. My guess is that it's still lighter-weight & simpler than d3 which makes it good for some use cases but not others.

Edit: I've only had good experiences with chart.js (not sure if that was clear)