The human interstellar nations
The Human Union is an alliance of four nations. These consist of the Federation of Hania, The Republic of Alcon, The Human Union, and People's Sovereign Union of Planets. Each greatly different from each other and to the Abh Empire. As one can imagine it can become very hard to keep a nation united when ruling over more that one planet. Sense the Empire only wishes to rule space it dose not try to force it's citizens to be patriotic to the Empire.
The People's Sovereign Union of Planets government system shares similarities with the Abh Empire. Each planet with in it's system has it's own military and general form of Independence. They have given up on the idea of having each planet share the same culture.
While there is the Human Union who force each system to share the same culture with them. This is gained through trading. This is probably why Samsonn's home planet never allied them selves with the Human Union but instead the Abh Empire.
Then there is the Federation of Hania, most of their planets already share the same culture so it was not hard for the Nation to become united. There are though, some planets with in it that has a culture greatly different from the majority.
The Republic of Alcon has many cultures under it, these have not yet been unified but because the main planetary system has a very established government it has been able to form into an interstellar nation. Compared to the others it is to some extent smaller to the other nations.