Functional considerations aside, there's definitely some personal issues with ligatures. Spacing in code is incredibly important for a lot of people. I'm dyslexic, so distinguishing between something like someValue=someOtherValue() and someValue->someOtherValue() adds additional unnecessary complexity compared to simply writing assignments with spacing (ie, someValue = someOtherValue()). This makes it significantly easier for me to interpret it at a glance.

That leads into my personal issue with ligatures: they change what should be a binary operation into a more complex one. Instead of "I see equals signs, one space or two?" it becomes "I need to stop, change to a vertical reading direction, and see if that's 2 or 3 lines" or "I need to estimate if that is 2 or 3 characters worth of width, or get off track comparing it to characters on the line above." Comparison operators should simply be "greater space equals" but become "wait, was there a second line there? I have to go back and double check that."

I'm sure someone will come along and say "how is reading the number of spaces different than reading the number of lines?" It might be that the spacing is normalized without ligatures, it might just be the vertical spacing in ligatures is too small, I don't really know. It's incredibly difficult to convey this to someone who doesn't experience it, all I can say is it is a night and day difference to me.

I've given Fira Code a shot 2 or 3 times but have found the ligatures to hinder my productivity every time. I also tried them just in the "reader mode" with Jetbrains Mono when that feature was first released, but had to turn it off after a few days.

That all being said, I won't fault anyone for using them if it helps them in the same way that not using them helps me. I'm just glad they can be disabled in Jetbrains Mono as it is otherwise an incredible font.