strncpy() does a specific thing really well. If it's the 1970s where you are and you're writing Unix filesystem code or some 1970s data processing application your program likely has a use for strncpy() and it matches the semantics you need exactly.
But it isn't intended as a solution for buffer overflow bugs in your program, and so if you try to abuse it to solve that problem you likely introduce more problems.
Imagine your car's air conditioning doesn't work properly. On sunny days it's really much too hot in the car. So, you buy a sunroof. Says "Sun" right in the name, surely that will help right? No. That's not what a sunroof is for. The sunroof works fine as a sunroof but that is not what you needed.
But it isn't intended as a solution for buffer overflow bugs in your program, and so if you try to abuse it to solve that problem you likely introduce more problems.
Imagine your car's air conditioning doesn't work properly. On sunny days it's really much too hot in the car. So, you buy a sunroof. Says "Sun" right in the name, surely that will help right? No. That's not what a sunroof is for. The sunroof works fine as a sunroof but that is not what you needed.