Etrata, Deadly Fugitive functionality was entirely broken if the face-down card is not a permanent.

[Grath]

I had a cloaked Anguished Unmaking that I had stolen from my opponent; I used the ability given to it by Etrata, which exiled it, cast it without its abilities doing anything, and put the Anguished Unmaking into play as a non-creature Instant. From the game log: 9:25 PM, T15.ET: Grath activates: Turn this creature face up. If you can't, exile it, then you may cast the exiled card without paying its mana cost. from face down object [ece] 9:25 PM, T15.ET: Grath moves Anguished Unmaking [ece] from battlefield to the exile zone 9:26 PM, T15.ET: Grath casts face down object [ece] targeting Coercive Portal [521] from exile zone 9:26 PM, T15.ET: Grath puts Anguished Unmaking [ece] from stack onto the Battlefield