It's been many years since I worked at a MSSQL shop but at least at the time (2008R2) there were issues with MERGE upsert statements deadlocking with themselves when run concurrently.

Based on https://dba.stackexchange.com/questions/187776/merge-deadloc... it seems that's still an issue as of 5 years ago, and not the recommended way of doing an upsert on MSSQL.