I once came across a true monospace ligature font. It was the absolute worst. Especially when combining letters like ff, fi, ffi...

Exactly. I recently started to write some LEAN which uses a ton of Unicode characters and I still use JetBrains Mono with ligatures on. ≠ is 1 unit wide but != is 2 units wide, so it's not that confusing as one might think.

Besides, if someone insists, it's also possible to make a language-specific setting in the editor to disable ligatures in those languages :)

On the other hand, this is one aspect that makes those ligatures typographically jarring to me. It replaces one compromise with another bad compromise.