Advantages:
The biggest advantage of an independent country, or a country that is able to get its independence, is freedom. They are free of the oppression of another country.
No one is in control of the population and an independent country can create its own fate, its own laws, decide how people want to live and make their own decisions on these topics.
Population can follow their own traditions and base its laws and way of living on it, not forced by another culture.
Disadvantages:
Gaining full freedom and independence can also bring some disadvantages. Relaying on the "oppressor" to lead and move a country and suddenly being able to start from scratch and lead your own people can backfire and bring chaos.
It can be hard for the people to create and start living under a new government, new laws, etc.
Examples: El Salvador, Costa Rica, Nicaragua, Honduras, Guatemala.
These countries got their independence from Spain. To be under the control of a country like Spain and then gain independence made all of these countries start from zero, which can be proven on the lack of order, political and economic issues, population chaos, etc. These countries gain freedom but they are still building a good democratic country.