Check and balance is still effective when one political party controls all three branches of government only if the three branches are separate and independent. Check and balances reduce mistakes and it makes power limited, controlled and defused. US democracy is strong due to its principles over party. Government should work to help everyone, regain the trust in an independent press and not do political corruption. Soviet Union had checks and balances but it became irrelevant under a one-party system.
Check and balance states that the independence of the three branches of government are not absolute, and these three branches of government are not watertight compartments.