This has never been a big problem for me. Some clients have strict policies (e.g., only allowing you to work on their servers) while others don't care as much.

This has never been a big problem for me. Some clients have strict policies (e.g., only allowing you to work on their servers) while others don't care as much. In some cases, as a freelancer, you only work on a "proof of concept" prototype assuming you have access to their private data. It is their FTEs job to actually train/implement/deploy this prototype using their private data.