Yes, it is generally safe to use a WiFi extender. A WiFi extender does not introduce any security risks to your network if set up correctly, and it is not inherently less secure than your existing WiFi router.
However, it's important to ensure that your WiFi extender is configured with strong security settings, including encryption and password protection, to prevent unauthorized access. It's also important to keep the firmware of the extender up to date to ensure any security vulnerabilities are addressed.
Additionally, it's worth noting that extending your network range may increase the risk of interference and signal interception, especially in public areas. It's important to be aware of potential security risks and take appropriate measures to protect your network and data.