I believe the reason Yakima says this particular rack is incompatible with the i3 is because it hampers the side curtain air bag system.
But I'm curious about it in actual use...
I bought a SeaSucker system back in September. I compared it to a competing product called VacuRack, which required straps to go under the door sill, sort of like this Yakima rack.
But I found complaints from users with cars that have frameless doors, similar to the i3, who complained of deformed door seals and even broken window glass, because of the straps.
In your day-to-day use of this, have you noticed any issues with the strap / door interface?