All PCB design is challenging, mixed-signal or not - let's clear that up right away. Doing a design from start-to-finish means a slew of steps and decisions, mixed in with trade-offs and compromises finally resulting in a finished product that meets (hopefully!) the requirements you inherited.

A mixed-signal design is any design that has both analog circuits and digital circuits on a single printed circuit board. In real-life applications mixed-signal designs are everywhere, for example, a smart mobile phone.

-E3 Designers (mostly Wikipedia)

Truth be told, you'd be hard pressed to find anyone NOT doing a mixed-signal design these days - that's just the way technology has evolved. At the very least you can expect to have to do some sort of power conversion - so whether you're looking at doing a PCB design as a maker, or if you're looking to become an electrical engineer professionally this series will give you a good look at the kinds of challenges the lie ahead.