I wouldn't say necessarily "risky", it's more that it forces your hand when you wouldn't want to reveal an entity's creation time. Say you use these IDs for users of your site, and they're used in API queries / URLs etc., then it's trivial to know when a user created their account. Sure, many sites already expose this information, but not all of them do; what if you don't want it exposed? What if you consider that a user's seniority is nobody's business, that it could bias the behavior of other users towards them, etc.?