Rakan dashes get interrupted by all hard cc and not just displacements.

Both Rakan's W and E will be interrupted/cancelled by hard cc and not just by displacements like other dashes in the game. The normal interaction between dashes and hard cc that is not a displacement is for the dash to finish and any remaining time left on the cc to apply at the finishing location for the dash. Below are a few examples of how it regularly works: (sorry, I don't have finalized videos to upload at the moment).

Rek'sai is one of the most recognizable champions for how the cc regularly works. If Rek'sai uses his tunnel right as he is getting hit by cc he will continue all the way to the other end of the tunnel. Usually upon arrive at the other end of the tunnel the cc will have worn off or be very brief due to the length of time that Rek'sai was moving for. A good example that you can see in several of the LCS games this season is Rek'sai being hit by an Elise cocoon right as he enters his tunnel and continuing to finish his tunnel Rek arrives at the other tunnel and the duration on the stun will already have worn off. The easiest interaction to reproduce is a Caitlyn trap. If Rek'Sai takes a tunnel that passes through a Cait trap Rek will finish travelling to the end of the tunnel and will be rooted briefly in place at the end of the tunnel. Rakan on the other hand using either his W or his E will hit the trap and stop immediately without finishing the dash.

Other champions who's dashes work exactly the same as a Rek'Sai tunnel as this include: (As a note none of the champions below have Flash like dashes as these are coded differently)

Aatrox Ahri Kalista (AA hop) Akali Alistar Azir Corki Diana Ekko (first half of phase dive) Fiora Gnar Amumu Gragas Galio Graves Panth (Stun) Irelia Kha Kled Lee Sin Camille Lucian Renekton Braum Shen

And I'm sure I missed several. As far as I am aware the only other dash that can be cancelled like this is Yasuo's e and that was specifically called out by Riot when it was implemented and is almost not even noticeable as Yasuo's dash range isn't very long.

This interaction destroys Rakan's kit as it is entirely built around his mobility. I have tried to search for any statement by Riot to figure out if this was purposefully put in place and have not been able to find anything. This issues makes it feel horrible to play Rakan compared to many other champions because you get used to the regular interaction of finishing all dashes and it just doesn't happen for Rakan.