What are the disadvantages in teaching religious and moral education in primary schools?

Depends on how the classes are offered.

I think it would be nice to have a class on "World Religions". Teaching an overview of each religion and ways to respect each other.

As far as "Morals" go, it would be more effective as a class in "Objective Ethics". Where Morals tend to be "What we do" . . Ethics might be more informative in "What should we do". "Objective" weighs in with what might be socially acceptable behavior for all.

Usually, morals is something one learns in social interactions and from your parents.

The disadvantage is, not all in school have same beliefs, even though they attend the school.  I feel it is up to the parents to teach religion to their families. Deuteronomy 6:6,7, " These words that I am  commanding you today must be on your heart. And you must inoculate them in your sons and speak of them when you sit in your house and when you lie down and when you get up."

The disadvantage is, that what if you had a Catholic teaching the religious studies in a predominantly protestant school? Perhaps a Hindu teaching it, or a Buddhist? Which religion do you infect these young minds with? Do you really want a person from another religion teaching your kids their beliefs or do you want a person from your particular sect teaching them? The problem is, that is is 44,000 different sects of christianity in the world, and countless other religions with sects inside of them, which do you want to teach religion to your children?

As far a morals, I think that is really outside of religion. Morality does start at home, but should be reinforced in school. Respect as well should be shown for those that do not believe as the majority. A class in world religions and cultures is a really good thing to teach and instill, that the western world is not the only culture in the world, and not even the largest culture.