Why is editing the unit stats not working?

[King Beowulf]

I used to mod all the time but just came back to M2 after Empire and I went to change some unit stats and now it does not show up in-game. Anyone know why? And when I try the "medieval2.exe --io.file_first" thing it just after the loading screen it just exits the game and returns me to the desktop, it's all very confusing

Edit: on top of using the file_first method I tried going into the preferences and did [io]
file_first = true and yet after the loading screen it does exactly the same thing can anyone help me out please!? Also if it changes anything I am using the Ultimate AI (AI only download) mod

[Gigantus]

1. It always is better to use the [io] file_first = true method in the cfg of your mod
2. You would have to give details of what you have changed so people will be able to help
3. Did you check your log if it gives you a detailed error message?