I'd say in Rakans case the uniqueness comes more from his playstyle as he is essentially a support with an assassin playstyle,something that no other support in the entire game has.
Also Xayah laying her feathers on the ground and being able to recall them all at once is rather unique.
Not to mention that they are the only 2 champs with inbuilt synergy.
Sure they don't introduce some major new mechanic(like Zoes spellthief,Ornns ability to make his own items or Kayns transformation),but they,like all other new champs(ivern jungling without killing,Camilles no-escapezone&physical shield,kleds 2 healthbars,etc.),still introduce some new mechanics.