Among all of the mechanical locking devices, castellated nut with a split pin is likely the most complex one and it will typically be used on the more complex, high-tech, usually expensive assemblies. Implementing it correctly requires a certain amount of skill and precision (unlike with the double nuts or spring washers for example).
Apart from tightening it to the correct torque, precise alignment is needed for the split pin to go through. Regardless of the torque, the gap between the threads still remains, allowing for micro-movements under vibration once the assembly is in service. While castellated nuts are the most expensive among mechanical locking devices, and certainly not something you’ll want to have replaced too often, you will need to keep inventory of various sizes and types for regular maintenance and replacement when some of them fail or corrode. And they inevitably will because the gap between the threads allows for the moisture to get in.
With a liquid threadlocker, all of the mentioned issues can be avoided. You just need to choose the correct grade in terms of strength you want to achieve (low, medium or high), and a medium strength product normally covers a huge percentage of applications, and you can use it across wide range of your needs. A liquid product will also completely fill all gaps between the threads, sealing the assembly against moisture and preventing any type of movement under vibration.
That way your threaded assembly will stay locked for as long as you want.