What makes FF Legato so unique?

"though I’m not sure how it’s a revolution in terms of readability"

I think the jury is still out and may be for some time.

What Bloesma did was execute a face that fit his personal philosophy and used his own skilled eye to produce it. We can spend days discussing success or failure to meet either readability improvement or even shake the chirographic mould. I don't know if we can come to a definitive agreement. Those who place Bloesma on the highest pedestal will convince themselves in the positive and those who are most skeptical will say nay. I personally see it as a successful typeface that is very well drawn and spaced but don't see it as anything earthshaking. I am not saying "the emperor has new clothes" but that he is not finer dressed than several other well drawn sans afoot.
I don't see how chirography is even an issue. There are both good and bad faces done using typical stroke tool mentality. I think the more successful faces do a better job of fitting glyphs together as words regardless of stroke pattern, stress, or dependency on figure vs ground. I am a firm believer that figure and ground must form a more perfect union and give and take prominence to affect that union. Weight plays a larger issue in dominence as well. Bolder weights make ground more prominent than lighter weights. To this,Legato is a bit heaver than the average face generally.
The rotations and shifts made their presence pronounced during the PoMo deconstruction period. This is a natural extension but with a purpose to reconstruct a new form. I think Evert has done quite well with it but not reached the moon. Too bad he did not live long enough to push it where he intended. He may have found something extraordinary but we will never know.

ChrisL