I bought a Cyclocross bike with Di2 and hydraulic Shimano brakes this summer. Spent a couple of weeks cycling very hilly courses on the French Riviera with 28 mm 4 Season tires. I just loved the extra confidence and sense of control the hydraulic brakes gave me compared to rim brakes. Both rim and disc brakes will lock up your wheels but hydraulic brakes have much better modulation. (and the 28 mm sligthly wider tires probably helped as well compared to my regular roadbike.).