Thanks for the heads up
What problems do those maps create? Just not placing the new resources?
Not exactly. What happens is that when you call GetMajorResourceQuantities (or whatever its name is), the routine is expecting one argument per strategic resource. So the function will return "1,1,2,1,3,4,2,1" and so on, depending on your game's settings (like the Abundant or Sparse settings). Those three maps I listed replace these functions with customized versions for that map, which means they'll only set values for the same number of resources as in the core game, with the assumption that the two lists line up correctly.
If you add new resources, then that routine (as well as the corresponding one for Small resource deposits) will have more arguments. In many cases of this sort of thing the call will now fail outright as the calling routine is expecting X numbers to be passed back, while the routine it calls instead passes back Y. If it doesn't fail, then an array overflow error like this will fill those X-Y variables with either 0's, or in some cases, whatever values happened to be in that particular memory address. That last possibility is the dangerous one.
This assumes, also, that your custom resources are always placed at the END of the list. If you'd added additional ancient-era resources and wanted them listed before Coal or Oil, things can get screwed up even worse. But let's ignore that for now, since you can avoid it. The problem is that if there are 0 deposits of a resource on the map, things will break. This is why AssignStartingPlots, at the very end, adds a final override for each resource that places 1 deposit if there aren't any. Assuming you've added similar overrides for your custom resources, the map might still be playable; if you forgot, then the map generator will crash before it finishes, and certain parts of the map won't be populated correctly.
In my own mod, I'd added more robust logic than that. Instead of adding 1 deposit of Iron if there were no Irons on the map at all, I reduced the chance of Iron being placed during the random distribution logics, and then at the end automatically added something like N/2 small deposits and N/3 large ones (where N is the number of civs). That way, even if the random part failed entirely, there'd still be a playable amount of Iron on the map. (I did this for every strategic, actually.)