The terms agreed at Brest are the whole of the matter. The Orthodox bishops of the Polish-Lithuanian Commonwealth who entered the union accepted the jurisdiction of the pope and the Catholic doctrinal position, and kept everything else: the Byzantine liturgy in Church Slavonic, the Julian calendar, communion under both kinds, their own hierarchy, and a married parish clergy. What they were offered in return was equality with the Latin bishops in the Commonwealth, including seats in the senate, which they did not in the event receive.
The union did not carry the whole church. Two bishops refused, a substantial part of the nobility and the lay brotherhoods opposed it, and an Orthodox hierarchy was restored in the Commonwealth within a few decades. The result was two churches using the same rite and the same language in the same towns, which made the difference between them a matter of allegiance rather than of appearance, and made the seventeenth century in those lands bitter.
The church survived because of where the border fell. When the Commonwealth was partitioned, the Greek Catholic communities that came under Russian rule were absorbed into the Orthodox Church, in stages ending in 1839 and 1875. Those in Galicia came under Austrian rule instead, where the state raised the standing of the clergy and funded their education, and the church there became the principal institution of Ukrainian national and cultural life in the nineteenth century.