Do relational databases really give you consistency? They can do if you are careful how you write your app. And many people don't use serialisable transactions

Do relational databases really give you consistency? They can do if you are careful how you write your app. And many people don't use serialisable transactions because of the implications. Also relation databases enforce their version of consistency. What if I only want consistency within one customer's data but not all customers? I could have two inserts for operations on two different customers fail because of a deadlock on page slips in a secondary index. I didn't need that consistency but the database didn't know that.