hive.merge.cardinality.check=false is a bad idea. The logic controlled by this property checks if the ON clause of your Merge statement is such that more than 1 row from source side matches the same row from target side (which only happens in WHEN MATCHED clause). Logically what this means is that the query is asking the system to update 1 existing row in target in 2 (or more) different ways. This check is actually part of SQL standard definition of how Merge should work. You either need examine your data or the ON clause but disabling this check, when it throws a cardinality_violation error, may lead to data corruption later.
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