What are the restrictions of using SEQUENCE object?

Answer:

There are some restrictions that one have to be aware of when using a SEQUENCE:

1) Unlike an IDENTITY column, the sequence values aren’t protected from UPDATE,
and the column that received the value can be modified.

2) There aren’t any unique constraints on a sequence value.

3) In a table, the sequence value may have gaps, because of a rollback or a SQL
Server service restart.