Been to Universal a bunch, and have ridden the buses, but not to Endless Summer.
But there are sidewalks from Endless Summer to Universal. You have to cross I4 on a huge/long/wide overpass (and be on the correct side), and the walkway appears to go right to where the buses drop off guests.
I'm 90% sure the buses are all wheelchair friendly, just like at WDW.
And there is a different accessible walkway from Cabana Bay/Aventura to Citywalk.