has it ever been criticised?
Yes, there is some public critique regarding its accuracy and potential bias.
Nielsen ratings
There is some public critique regarding accuracy and potential bias within Nielsen's rating system, including some concerns that the Nielsen ratings system is rapidly becoming outdated because of new technology like smartphones, DVRs, tablet computers and Internet streaming services as preferred or alternative methods for television viewing. In June 2006, however, Nielsen announced a plan to revamp its entire methodology to include all types of media viewing in its sample.

Since viewers are aware of being part of the Nielsen sample, it can lead to response bias in recording and viewing habits. Audience counts gathered by the self-reporting diary methodology are sometimes higher than those gathered by the electronic meters which eliminate any response bias.
family guy - peter beats up chris's bully season 5 ep 11 UNANSWERABLE what is the tan aquatic with steve zissou? It is the eleventh episode of series "Family Guy". how was it received by the people? The episode was viewed in 8.53 million homes in its original airing, according to Nielsen ratings, Ahsan Haque of IGN gave rating 7.4/10. what is this ratings system? "Nielsen ratings" are the audience measurement systems operated by Nielsen Media Research that seek to determine the audience size and composition of television programming in the United States using a rating system.
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Nielsen ratings [SEP] Criticism of ratings systems
Another criticism of the measuring system itself is that it fails the most important criterion of a sample: it is not random. A small fraction of the population is selected and only those that accept are used as the sample size. In many local areas during the 1990s, the difference between a rating that kept a show on the air and one that would cancel it was so small as to be statistically insignificant, and yet the show with the higher rating would survive. In addition, the Nielsen ratings encouraged a strong push for demographic measurements. This caused problems with households that had multiple television sets or households where viewers would enter the simpler codes (usually their child's) raising serious questions to the quality of the demographic data.