Modding is fine. Cheating isn't.
I tend to look at this from a slightly different perspective: If I don't like the rules of the game, then I change them.
But I agree that providing one player in a PvP arrangement with unadvertised buffs/nerfs is different from say a golf handicap.
In Stellaris, creating a player race involves spending a limited number of ethics and civics points. But with a large collection of mods that add new and interesting ethics and civics, the points given to work with no longer seems appropriate. So I upped those allowances.
In Rimworld, the time spent mining a resource is a factor, so the play is to mine the highest profit/time item and sell it for silver to buy the resource I really want. Which seems quite peculiar when I can mine that resource. So I changed the profit/time to be identical across resources; this way I can choose to mine what I need instead.
In Minecraft, the Minecolonies mod introduces a tech tree mechanic where I pay resources to discover new tech. But there's a real-time progress bar that simulates researching the tech. Since I don't find progress bars entertaining, I eliminated that part and just pay the researchers for techs I want to buy.
