Most Western Vocaloid fans are largely unaware of any problems with Vocaloids from Japan. However, there is a lot of fan favoritism in this area. Some of the most commonly cited problems in Japanese Vocaloids are the Kagamines' poor enunciation and Luka's English (as well as Lily's voice in general) among western fans.
However, bear in mind that as they are more popular, a given problem will be more likely to be observed in a popular Vocaloid. Keep it in mind that both AH-Software and Internet Co. have made Vocaloids on par with Crypton's. Even if a Vocaloid is of better quality, it does not factor in how easy the Vocaloid is to use. In particular, as Crypton's Vocaloids are the most popular, more information on tuning them is publicly available than with other Vocaloids.
Any statement that one studio is better than another requires more definition of what is meant by best -- the one with the most profits, the most individual sales, the highest-quality voicebanks, the most accessible voicebanks? No matter how good Miku is, she isn't much use if you want a male singer for your song.