Bitter lesson in research that I had to learn a few years ago: Topics go in and out of fashion, and sadly that matters.

For example, when I started doing research, I wanted to work on wireless mesh networks.
However, my supervisor said, Sorry, that topic was popular 10 years ago. Not going to get published, we don't have the expertise here, ... work on something else instead. Like Software Defined Networking, which went on to become the new fancy research buzzword. :/