The Ultegra R8000 series Di2 RD comes in both short & med. cage styles. Max for the short it 30T & med. is 34T although Shimano is notoriously conservative about this. As long as you get a bike with the med. cage RD you're good to go. A lot of endurance bikes now come with an 11-34T but if it doesn't as long as the RD is a med. cage a good bike shop will swap out to the 11-34T as part of the sale and setup.
The Dura-Ace series R9150 Di2 RD only comes in the short cage style so 34T might be more problematic.
I love Di2 as do most people who have it but there are few who prefer mech. or would rather spend their money differently.