In relational databases, data integrity is ensured through ACID properties, which guarantee atomicity, consistency, isolation, and durability. Non-relational databases like MongoDB lack these properties, making it challenging to maintain data integrity, especially with multiple operations. However, MongoDB 4.0 introduced multi-document ACID transactions, allowing developers to write code that can start a transaction, modify multiple documents, and commit changes while maintaining integrity. While this feature eliminates the major drawback of non-relational databases, heavy transaction use may require changing the data model for optimal performance.