I work with someone who uses them and I find it impossible to read his screen. Unless you work completely alone, it's a terrible idea.

*Unless all people that might ever look at my screen are used to them.

If you work with people over the internet, it doesn't matter because they can disable ligatures and still read your code. And obviously it's not a problem if people know them and are looking on your screen.

But yeah, if you're doing pair programming and first dev is used to them and the second dev is not, that's a problem. The question is, is the solution that the second dev learns them or that the first dev stops using them - both will obviously dislike changing the way they like to code...

Can't guarantee 80 characters length easily.

This isn't a problem with ligatures that are designed to take up as much space as the characters they replace. Fira Code seems to have been designed with this in mind, so for example, the === ligature is exactly three times as wide as a normal character.