The most common criticism I'm aware of is that it leaks information about your system and its users. For example, you can gauge growth by how fast ids grow, and

The most common criticism I'm aware of is that it leaks information about your system and its users. For example, you can gauge growth by how fast ids grow, and relative age of accounts. The former is potentially financially valuable. Often the latter is exposed properly through the UI, but the that can be a choice, rather than exposed due to an implementation detail. There are likely other criticisms as well.