The only major downside I see to this is preserving and quick retrieval of knowledge. It's a well known fact that many luminaries over the course of several centuries had systems similar to a Zettlekasten, most notably, Niklas Luhmann, the prolific sociologist apparently had several thousand index cards preserving info.

While it's commendable, it should also be noted that all this was done prior to the information age we're living in. These are physical objects that at the end of the day take up sizeable space in your house, gather dust, need cleaning/dusting etc. And you can't throw a computer at the problem of search and retrieval, at least yet.

I wish there was more research into active learning by typing as well. Typing does force you to think, especially if you don't type something you learn verbatim but actively try to process info and possibly summarize it in your own words.