What are the potential downsides of using the UPSERT operator in databases?

The disadvantages of using the UPSERT operator may include:

• Potential performance issues, especially with large volumes of data, as the operation may require checking for the existence of a record first, then inserting or updating.
• Possible conflicts when multiple transactions access simultaneously, which can lead to locks or errors.
• Not always supported equally by all DBMSs, reducing code portability.

It is important to assess how suitable UPSERT is for a specific task and database, and to optimize or use alternative approaches if necessary.