What are the down sides of using a composite/compound primary key?

Question

What are the down sides of using a composite/compound primary key?

Answer 1

1. when you se it on a diagram are less readable
2. when you use it on a query join are less readable
3. when you use it on a foregein key you have to add a check constraint about all the attribute have to be null or not null (if only one is null the key is not checked)
4. usualy need more storage when use it as foreign key
5. some tool doesn't manage composite key