I write so because my best friend knows two languages: Albanian and Italian.
That's true that being bilingualism is beautiful, because you learn two languages without studying a lot, but there are negative sides, too. For example my friend doesn't know very well neither Italian, nor Albanian. It is like she doesn't have a native language. And also in other cases I could see that who can speak two languages, knows well only one of them.
what is your opinion about?
I don't think anyone in their right mind would ever call bilingualism 'bad'. If a child gets equal exposure to both languages growing up and is encouraged to use both on a daily basis (and I don't mean only orally) there's no reason why one should be better than the other.
I grew up speaking French and Spanish at home and English at school (from age 4) and I feel comfortable using all three languages but I was always encouraged to read a lot in all of them and I know it helped immensely.
Thank you for your comments!