So in these systems, a fork can happen which means two or more different blocks can be made on the same height. Usually "Longest chain wins" rule is used to solve the fork condition. It means that those forks will be merged into a single canonical chain eventually, but it also means a list of blocks can be reverted in some period of time when it belongs to the shorter chain. So in these algorithms, there is no guarantee of the finality of blocks and transactions. The finality only can be achieved probabilistically after some period of time though it can't be 100% sure.