Pre-observable, I used d3 to develop a very complex visualization with real-time updates. It was difficult because d3 has a lot well-thought-out-but-complex concepts, but the docs and examples helped me fully understand how it worked, and the result turned out spectacular.

A few years later, I needed to build a simple novel visualization. A major new version of d3 had come out, and all the examples and documentation got Observable-ified.

Enough time had passed that I could only vaguely recall some of the original concepts, so I set out to re-learn. YMMV but, for me, it was impenetrable. With the limited time I had, I just couldn’t figure out how to untangle d3 from observable. I gave up. Very sad.