WASHINGTON — Several of the key surveillance reforms unveiled by President Barack Obama face complications that could muddy the proposals’ lawfulness, slow their momentum in Congress and saddle the government with heavy costs and bureaucracy, legal experts warn.

Despite Obama’s plans to shift the National Security Agency’s mass storage of Americans’ bulk phone records elsewhere, telephone companies do not want the responsibility. And the government could face privacy and structural hurdles in relying on any other entity to store the data.

Constitutional analysts also question the legal underpinning of Obama’s commitment to setting up an advisory panel of privacy experts to intervene in some proceedings of the secret Foreign Intelligence Surveillance Court, which oversees the NSA’s data mining operations. Obama has asked Congress to set up such a panel, but senior federal judges already oppose the move, citing practical and legal drawbacks.

The secret courts now operate with only the government making its case to a federal judge for examining someone’s phone data. Civil libertarians have called for a voice in the room that might offer the judge an opposing view.

“The devil is in the details of how the government collects and retains phone records,” said Anthony Romero, the executive director of the American Civil Liberties Union, “and I think we’re going to see pretty quickly the lack of specificity behind some of the president’s promises.”