you have to also consider though that it was just after age of strife (?) So everything was quite bad everywhere they dint have solid empire behind them just a handful of planets. also it was a whole dam galaxy and I'm sure there way more planets in it then a thousand. Also as you said the tech was more advanced so the enemies they were facing had somewhat equal tech or better. Also as you see aftet his death those world fell to instability but during the great crusade they were not only conquering but expanding the empire, building and expanding the new infrastructure.