DuckDB is similar as an in process SQL database, but lacking btree-style ordered indexes makes it a poor performer in key lookups and order-by / range scans if your table is any size larger than trivial.

It’s the classic OLAP (DuckDB) vs OLTP (SQLite) trade off between the two. DuckDB is very good at many things but most applications that need a traditional SQL DB will probably not perform well if you swap it over to DuckDB.