Hydraulics take a lot longer to actuate and are a lot less efficient than electromagnetic. The mazda system allows for better/quicker response. The ea888, american version (not dual injector setup), requires walnut blasting. I've seen recommendations from 40 to 80k miles, depending on conditions. Point is...its an issue that doesnt plague SkyActiv engines. GM and BMW n54 also have similar issues as the VAG engines. Mazda took great pains to run the intake valves HOT, as well as an excellent PCV system, so that their DI only engines don't have this issue.