I have yet to run Shadowdark, but it seems like a perfect way to introduce D&D 5E players an oldschool dungeon craw campaign. There's familiar mechanics (dis/advantage), the old percentile Thief skill tables are gone, and DCC-style "roll to cast" seems like it'd be more palatable for folks used to attack Cantrips and ample spell slots. Plus, Kelsey Dionne is a fantastic adventure writer, so we're all but guaranteed years of top-shelf modules.
Yup, unified resolution mechanics tend to be easier for new players to grasp. (I suppose a good comparison would be if Call of Cthulhu had you bust out a d20 to resolve breaking and entering.)
Shadowdark (like White Box: FMAG) replaced the d100 thief skills table with a simple, "Roll advantage on Thief type ability checks." It's a personal preference thing, but I like it.