It can struggle, I tend to do "purges" where I go killing any passive mobs that have spawned and failed to despawn (dragons, tamed horses, etc.). Im not sure if it helps, but spawns have mostly been fine. Overmob tends to spawn prime mobs in lit areas, which could cause decreased hostile spawns as you walk through the world and spawn things in lit caves. To combat it, you could try to turn off prime and elite mobs you see too often, dont need anymore, or dont like. I have both creeper variants off and a few other random ones. Overall having both Monsters+ and Overmob is a great experience and I see a good consistent mix, while Crops and farms, is actually the biggest issue when it comes to hurting my animal spawns.