One notable problem with user ids is that they tend to leak to users in some way. Because of this, it's really easy for competitive intelligence companies to bo

One notable problem with user ids is that they tend to leak to users in some way. Because of this, it's really easy for competitive intelligence companies to bot create accounts just to get a feeling of new user growth if you're naively using auto increments. This maybe fine mom and pop sites, or sites where somehow the UID never leaves the middleware, but the reality is that using sequential IDs or hashes of them without salt aren't great for this reason.