What the disadvantages are in becoming a librarian depend a great deal on why a person wants to become a librarian in the first place. In other words, the disadvantages seem to lie mostly in individual expectations that may not be met. There are educational and financial aspects to becoming a librarian that are problematical today, as well as possibly some unmet expectations regarding job satisfaction.
Today, it is almost impossible to become a librarian without a master's degree, which for most people means student loans for approximately six years of education. That is a long time and a large investment for a profession in which the average starting salary is not quite $57,000 per year. Furthermore, this is an area in which growth is predicted to be slow, about 2% a year through 2024. Becoming a librarian is not the road to monetary riches.
For the person who loves old-fashioned books and wants to spend the day talking to people about them, being a librarian today could be a disappointment. The offerings of libraries are increasingly digital, with a far greater emphasis on music, movies, and even video games. A librarian must now be technologically proficient and spend far less time making book recommendations and chatting about good books. Probably the closest one can get to the old-fashioned kind of librarian is in a very small town library or in the children's section, where parents sometimes do really seek advice for their children's reading. And story hour is any librarian's delight.
All of this is not to say that becoming a librarian is a bad idea. It is just to say that the expectations of the profession's rewards must be reasonable ones. Most of the librarians I know, and I know quite a few, are perfectly happy people.