Here are the sentences:
Her latest novel is her best yet.
It’s one of the toughest warnings yet delivered.
Replacing yet with ever, how does it feel different?
Her latest novel is her best yet.
Her latest novel is her best ever.
I think the meaning is the same there because of the modifier her latest (that is, ever and yet would both be understood to mean "so far"); but ever is in a more informal register and a bit gushy whereas yet is neutral.
It’s one of the toughest warnings yet delivered.
So far, in the current context or situation, this has been among the toughest warnings.
It’s one of the toughest warnings ever delivered.
Absent further context, it is one of the toughest warnings delivered in any situation at any time.
"Yet" means up to now while "ever", well, means ever (which can also refer to the future). So the sentence
"Her latest novel is her best yet."
means that the writer is still alive and might write a better novel but up to now it is the best novel she has written. But if we say
"Shakspear's best play yet is Hamlet."(WRONG)
it would be meaningless since he is dead and is not able to write any more plays. So we would have to say "Shakspear's best play ever was Hamlet."