From open source information in books about the topic, an example of the issue would be foreign intelligence bribing or blackmailing a source, and then having to internally weigh the credibility of the information.

Opposed to many countries having citizens rights about trial and facing accusers, so information gained because they threatened someone about exposing an affair, is not considered okay in court.
Foreign intel does not need to convince a jury with a judge and defense attorney. Credible information can be "Well we bribed a food company dock worker and know their ships just loaded enough food for 3 months."

It's one of the issues that is cited in the intelligence failure before 9/11.
Foreign intelligence didn't want to work with Domestic intelligence because domestic intelligence would need to explain in court where the information came from. Also because they didn't trust them to maintain security on the info. Also because there is a higher duty to act domestically. Foreign intel may want follow something and see where it goes to work the entire network, while domestic might want to make an arrest. Domestic didn't want to work with foreign for the inverse reasons. Getting info from something that would violate rights domestically means info is unusable in court. Foreign side wouldn't elaborate on sources either which is not enough for legal proceedings.