You may need either to do extensive data scrubbing or to fix the OLTP systems. Talk to users about their data needs, and decide whether the result justifies the cost. Security considerations for data warehouses are different from those for OLTP systems. For a data warehouse to pay for itself, lots of users have to be able to benefit from it, and therefore more users will need access to data than are traditionally authorized by OLTP security. If your organization can't make that cultural change, a data warehouse may be a wasted investment.
Data warehouses have to be user-friendly.
Since a data warehouse is not a production system, users don't have to use it. And the fastest, most reliable system won't be used if the user doesn't understand it or finds it cumbersome to work with. A data warehouse sometimes takes years to implement. Before embarking on a data-warehousing project, determine what your business objectives for it are, what the potential costs are versus the benefits, what resources you'll need, and how much organizational commitment you'll need. Without good planning and user buy-in, the project is not likely to be successful.