I think the author failed to state the primary issue I have with ligatures: code listing using ligatures tend to be harder for me to read because the symbols are unfamiliar to me.

I first ran into this issue trying to learn Haskell, where some beginner websites used fonts like Fira Code in their listing. The "weird" operators I saw in the listings were unclear to me. In some cases I simply wasn't sure how to even reproduce the code in my editor. I think it is something I could grow used to and someday perhaps even enjoy, but as a beginner it was a barrier.