A logic analyser is cheaper than a scope, and does a much better job displaying this kind of data in volume. I'd say a DMM + some equivalent to a Saleae Logic are the two tools I couldn't live without ... IF you ever have to write drivers. But so much of embedded is about interacting with other devices, and it's a common enough requirement to have to port a driver over to a new chip, etc., that I can't imagine anyone regretting buying one sooner rather than later.

You can get by with printf, clearly ... but an analyzer is worth it's weight in gold for the right problem.

Dealing with anything in the GHz range is not only extremely expensive (order or magnitude more), but you also start to deal with far more complex problems that boil down to the need for a very good understanding of the underlying physics of signal transmission: concepts like impedance matching, crosstalk between signals on the PCBs, etc.

The design AND debug requirements are far more complex, and you need to account for a lot more explations of why something isn't working as expected ... and the software/firmware AND physical level.