I initially liked them, but later changed my mind. The same goes for e.g. prettify-symbols-mode in Emacs or conceal in Vim, which takes the same concept further.

I often work with files (both documents and code) where Unicode input is permitted, and it can then be important to know what exactly is written in the file. Fancy ligatures make this ambiguous.

I would also advise against ligatures in terminals, as it can mess up some text-based interfaces.