European privacy regulators are calling for changes to a commercial data transfer pact between the US and EU.

The Article 29 Data Protection Working Party (WP29) today said the proposed Privacy Shield agreement, meant to replace the invalidated Safe Harbor deal, is not clear or robust enough.

Established 15 years ago to ensure businesses treat data moving between countries with the same privacy protections as inside the region, Safe Harbor came under fire following mass surveillance revelations in the Snowden documents. In October, Europe's high court ruled that it was invalid.

Both sides have been trying to hammer out a better arrangment for two years, though. An agreement, known as the EU-U.S. Privacy Shield, was announced in February. The program imposes stronger obligations on U.S. companies to protect Europeans' personal data. That means organizations like Facebook must display their privacy policy online, reply promptly to complaints, and self-certify annually that they meet the requirements. European citizens, meanwhile, can expect more transparency and responsiveness.

"Our first reaction was very positive," WP29 Chairwoman Isabelle Falque-Pierrotin said during a Wednesday press conference, commending the nations on the "important improvements" compared to Safe Harbor.

Upon closer inspection, though, the regulatory group found the new pact too complex and difficult to understand. "We believe that some key data protection principles, as outlined in European law, are not really flected … or have been inadequately substituted by alternative[s]," Falque-Pierrotin said.

Today's announcement marks the WP29's final assessment of Privacy Shield. Though the group's opinion is not legally binding, the chairwoman said she hopes the governing bodies take their concerns into consideration. As the New York Times reports, opposition to any deal could set it up for future legal setbacks.