Constraints are considered a Good Thing
. They enable to rely more
heavily on the validity of the database. It is quite important to note that
validity is in terms of modeling and not in terms of
business [1].
The biggest complains we can have about constraints is that it is sometimes quite annoying to do some updates while consistently validating the constraints. You have to care about the order of the operations you do.
Notes
[1] I'll write an article about this later