Undo via ephemeral toast seems like a bad idea also. I guess if the toast hanged around until the next toast it might make sense?
In other words, if you delete an email and it goes to the trash folder: good use of toast + undo
If you empty the trash, and there is nowhere the user can go to unempty it: bad use of toast + undo
Its also useful as a sleight of hand eg when cancelling an action you havent yet taken (which actually is generally what a toast + undo actually is). The best example of this being toast + undo for an email send