SIP ALG (Session Initiation Protocol Application Layer Gateway) is a feature that is commonly found in routers to improve the functionality of VoIP (Voice over Internet Protocol) traffic. However, SIP ALG can actually cause more problems than it solves.

Here are some of the reasons why SIP ALG is considered bad:

1. Compatibility issues: SIP ALG modifies SIP packets in transit, which can cause compatibility issues with some VoIP systems. This can result in poor call quality, dropped calls, and other communication problems.
    
2. Security risks: SIP ALG has been known to cause security issues, such as opening up network vulnerabilities and exposing sensitive information to unauthorized parties.
    
3. Configuration problems: SIP ALG can cause problems with proper network configuration, such as incorrect port mapping and NAT traversal issues.
    
4. Lack of standardization: SIP ALG is not standardized, meaning that different implementations can have different levels of compatibility and functionality.

For these reasons, it is often recommended to disable SIP ALG on routers to avoid potential problems with VoIP traffic. If you are experiencing issues with your VoIP system, disabling SIP ALG is often the first step in troubleshooting the problem.