Had I known the pop of 66's was so low for the Bust Dimes I would've stayed at 65+. NGC is very variable on grading of foreign. Just got in a Brazilian silver crown (2000 reis, early 20th century) which I bought at auction in an NGC-64 slab. Coin was a dog. Is ms but a lot of bagging on the obv which I thought would'nt be warrented on the smaller, reduced size silver crowns of this type. Often NGC's been undergrading Brazilian {IMO} but unfortunately not the case for this coin. Still it was cheap which I suppose is the only thing good about it. Also fills a hole in my type collection.
Every grading service makes mistakes in both directions. The trouble is when you buy an over graded coin in the holder, you are stuck unless you got for well below the catalogue value for the given grade.