Both are actual risks. If the other networks are not configured on appropriate channels then there could be RF interference with your clients/access points. This will reduce the reliability of the connection for your users and services.

To keep unauthorized users off of your network, you need to ensure that you have fully secured your network with the appropriate client authentication/authorization and encryption. You might consider working with a local reseller who has significant experience with WLAN implementation and support to ensure that your systems/services are protected and implemented according to your business needs.