This problem probably stems from the fact that at levels 25, 50, and 75, when you progress to a new difficulty level, the monsters get huge multipliers to everything: hit points, attack, defense etc. I had to tone this way down in my old DC mod. In DC, it just meant that some characters had a very tough time fighting monsters in the higher levels. In Drox, the player is much more powerful than in DC, and therefore it's not a matter of the player surviving, but of the races no longer being able to put up a fight against the monsters at the higher levels.
The Talon and other monster races have the advantage that their specific races don't attack them. That may be enough of an advantage to dominate at high levels. Additionally, it seems like they may be able to recruit monsters from their races, which, if correct, would allow them to recruit very strong monsters at the higher levels. These monsters may be making the difference.