What is there not to trust? There are lots of disassemblers for binary files. Ghidra just comes with tools to make analyzing the resulting assembly code easier by doing things like graphing the jumps in code, allowing the user to give custom names to variables and functions, and attempting to convert the assembly into C code.
It would make sense that the NSA spends a lot of time reverse engineering programs. Not all hackers share their exploits publically, so one way to find unpublished exploits is by reverse engineering viruses and malware to find out what vulnerabilities are being exploited.