In the US, the government is probably going to ask for (and receive) some sort of national security waiver to sharing the details. Generally up to the judge to determine if that request is valid.
Or, the law enforcement agency will use parallel construction to get a conviction without exposing their tech.
Now, true, the judge is probably not going to let the defense attorney, in closing arguments, say "That evidence came from a hack of the defendant's phone, which is a really invalid thing to do, so you should ignore that part of the evidence." Judges get really unpleasant with attorneys who cross the judge's lines on what they can and can't say in front of the jury.
So this would probably be left to the jury to figure out on their own. I'm not sure how likely that is...
Safe from potential pesky legal precedents.