There’s a potential problem even when law enforcement announces its presence.
I am practically deaf in my right ear. Because of inner-ear problems I sleep on the side with my better ear. An announcement at my front door might not be audible to me in my bedroom with the door closed, as it customarily is. My wife would hear, but not everyone has someone to serve that function.
No-knock raids in my mind violate the spirit if not the letter of the Fourth Amendment. They were originally “justified” to prevent destruction of evidence in drug cases IIRC; that is to say, they prioritized expediency over liberty. In recent times we have seen them used as much to terrorize civilians as to obtain evidence. They’re dangerous both to civilians and to law enforcement. There have also been instances of raids mounted at incorrect addresses.
Atlanta Drug Red Dog Squad killed a little old lady that way..