As it seems we agree: Sadly, this is, by and large, true. The more languages you learn; the more superficial your understanding of the subtle nuances of each is.

Yes. You can count on the fingers of one hand the number of good bilingual writers: Samuel Beckett, Joseph Conrad (thinking hard here...)

Meanwhile, Max moans: What irks me is the facility with which children raised in polyglot households prove themselves the exception to this general rule. I agreee with your assessment, and it seems to me that the only way to be truly multilingual is to be exposed to the languages in question from birth.

Indeed, as set forth in "Language Acquisition" under Miscellany After puberty your language-learning center shrinks.