I also don't know if Unexpectedly Absent is a strict upgrade to Azorius Charm. I wish more charms were cubeable at my level. They are fun to play with but it's always vaguely difficult to justify spending design slots on them.
part of the difficulty is that they were designed for constructed it seems. the 3-color charms are not worth it (though a solid 3/5 saw play?) and the 2-color ones break down as follows:
UW -- standard staple
BU -- fringe standard play
RB -- great modern board card
RG -- fringe standard play
GW -- standard staple
WB -- standard staple and Modern game-changer if I am in the tournament
BG -- incredibly good standard and legacy board card
GU -- saw a little play in Standard
UR -- standard and Modern staple and nearly good enough for legacy
RW -- modern and standard staple
so you'd want to just throw all of them (but UB UG RG) in your cube, but then you realize RB isn't at all maindeckable, GW isn't better than the other cards that cost GW, WB is lamer than (say) Sculler and Gerry's Verdict, BG isn't really maindeckable, and RW neither burns them out, gives a suited-up dude double strike, or protects said dude from wraths in Cube often enough. Moreover there is the difficulty of casting a 2-cost card on turn 2, which is easier for Cube decks here than anywhere else but still pretty hard.
for the most part, these are perfect examples of cards that are designed to fit into constructed, where their flexibility brings great joy. for cube though? if you're me you just end up having UW and UR.