Some maps like USA had a high success rate over 50%, others like Waterloo and Circus Maximus had success rates as low as 18%.
I added new recruit join filters on all maps with success rates below 40% (that means new recruits can still join games on 47 maps

I also filtered out some other game options that significantly lower the chance of new recruits making their promotion. See Instructions -> Score, Ranking and Medals for more details.
This should improve the success rate and new recruit experience (for most, as it is based on stats) and cut down on the "newbie farming" going on.