Well, I came here to see if I should purchase Old Gods and your fun games have tipped the balance out of my account and into Paradox Interactive's eager palms. Now I just need the friendly modders for PB, SWMH and VIET to update!

- Birth rates have been reduced a lot
That's probably a good thing. My last game as the von Pruessen Kings of Vendiland each king produced about 9 children, maybe more... Although I'd prefer it if rather they kept the birth rate the same, but instead increased infant mortality. In fact, I may just try implement that myself!

Also, the Horse Archer mercs are super-helpful, and I've found they can take down armies about 1.5 times their size with the right commanders.
Horse Archers are the second most powerful unit type, their stats mean they're approx half as powerful as knights (and 35% stronger than LC and a whole 78% stronger than pikes). The fact they have high skirmish numbers means armies are very susceptible to taking big loses initially, and then never recovering. In short, yes they're good!