When I played as the Julii, I got lovely rewards. About 80 percent were lovely gifts. 10k and 5k rewards for taking cities. All decent mercanary units. The worst mercanary unit I got from them were the numidian cavalry but they gave me 2 of them and since it's hard to train cavalry auxilia early they were in good timing. The other 20 percent were poor gifts but when they gave me missions for being considered into senate office it allowed my whole faction to be elected to senate and the missions seem quite easy once you have that power. When I had every senate position it seemed I'd get a mission to take a city I already had besieged. It does seem the more positions you have in the senate the easier the missions and the better the rewards.

Now when I played as the Scipii, the best unit I got from the senate was Scorpions. 1 10k mission reward and the rest were lousy rewards principes, ships and barbarian mercanaries and I never held more than 2 positions in the senate.

So it seems the more control you have of the senate the better the rewards are.