There are better adjectives that could have been used here, and in many other sentences. "-zuri" and "-baya" are overused. In this module on holidays, "Muungano" refers to the union between Tanganyika and Zanzibar. Assuming that, and that the course creators wished to say something positive about it, some adjectives that would have been more descriptive are imara, madhubuti, thabiti (all meaning strong), and possibly muhimu (important), or wa manufaa (beneficial).
It's most likely the union they are referring to is the union of Zanzibar and Tanganyika to form Tanzania. It's a public holiday on 26 April called union day. https://en.wikipedia.org/wiki/Public_holidays_in_Tanzania