NBCUniversal has “deep concerns” that Nielsen’s Total Content Ratings (TCR) — adding ratings for digital viewing of TV shows to the current numbers — are “not ready for release,” the Comcast-owned company told Nielsen in a letter sent on Tuesday, but that surfaced today.

The complaint from the No. 1 seller of TV ads could be significant: Networks have wanted the change as they try to persuade advertisers that they’re still a good buy at a time when traditional ratings are declining.

Nielsen says that it “stands behind” the TCR effort, which remains “on schedule to syndicate data on March 1.”

NBCU’s letter from Advertising Sales and Client Partnerships Chairman Linda Yaccarino says that TCR is “far from meeting” requirements that it offer “a reasonable degree of accuracy and transparency” — making its release “premature” and “irresponsible.”

“Some say, ‘something is better than nothing’,” Yaccarino writes. “We disagree. Bad, inaccurate and misleading data is far worse than no data at all.”

Among her complaints: TCR doesn’t include “leading Pay TV operators and digital video distributors.” It mixes different forms of audience measurement, including one from YouTube “that is not yet vetted.” It depends on “opaque Facebook data” to make sure that viewer counts aren’t duplicated. It has “no quality control measure for digital fraud” And it “fails to measure Out-of-Home viewing.”

As a result, Yaccarino says, “Nielsen’s specific promotion of TCR as a tool to effectively inform smarter business decisions including media allocation will be detrimental for all stakeholders — advertisers, agencies, publishers and content distributors.”

Nielsen says that it is still working to “enhance and refine our product with ongoing updates as we work with clients during this period of evaluation….We do not underestimate the importance of the rigor of this work, as clients use our metrics to make decisions on content, programming, distribution, and as a currency for ad planning and settlement.”

NBCU’s letter doesn’t appear to worry Wall Street. Nielsen shares are down less than 1% today.

TCR “was never going to meaningfully alter Nielsen’s dominant position in this business,” Pivotal Research Group analyst Brian Wieser says. Problems with the product “will eventually be remedied” — and what really matters are what advertisers, not networks, think. Few of them “have had significant exposure to the TCR product.”