I can't speak about the DAB protocol, but the HD Radio scheme that is used for digital in the United States seems to be a protocol problem. I say this because I've owned four HD Radios each from a different manufacturer, and each one had the tuning lag problem.

The sad part is that HD Radio is actually startlingly better on AM, while FM is an incremental improvement. But few AM stations use HD because most AM stations operate on a shoestring, and it ruins your fringe coverage.

But when you find an AM station that's in HD and you hear the receiver switch from analog to digital -- holy cow!