What is SQL Server not good at?

Defining constraints declaratively.

The CREATE ASSERTION statement has been in the SQL standard since 1992. Microsoft still haven't implemented it.

If you have a constraint that states "a department must not have more than its maximum number of employees" you currently have to write triggers on two different tables to achieve this.

With CREATE ASSERTION this constraint can be achieved with a single SQL expression.

As none of the other major SQL-DBMS vendors have implemented assertions, this would give Microsoft a major selling point.