Numerous explanations as to why she was actually booed (from low quality singing and lack of bel canto style to anti-American La Scala fractions) are discussed here. Renée Fleming herself has stated that she in retrospect probably took on too many new parts that season, and that she didn´t feel she was singing her best.
As I know nothing of bel canto, my opinion may count for very little, but apart from the somewhat strange ornamentation dividing the two sections of the aria (is that a high F?), I think she sounded just fine.
You may now judge for yourselves:
Renée Fleming: Era desso from Lucrezia Borgia. La Scala 1998.
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