depends on what level of isolation and how it's done, MVCC avoids deadlocks, but if you manually lock any row/table, you can still run into deadlocks. but the DBMS' transaction manager will notice and timeout one of the transactions, failing it.

While MVCC avoids deadlocks, you'll still get a lot of failed transactions if they all operate on the same data at the same time, which you'll have to retry, but I'll take that over data inconsistency