I've tried out a few of the factions.
Senones are really quite hard. They're unable to peacefully occupy settlements, meaning that any time you take a settlement you've got to raze all the buildings. Can be frustrating, especially when retaking your own settlements if you've lost them. But also makes for a thematic playstyle.
I got fairly far with a Syracuse campaign, though then lost the saves when my computer failed. The ability to draw upon additional mercenaries that aren't available for anyone else is nice, though the upkeep costs can be punishing. There are techs and skills to help with that though, plus you can also use popup armies (keeping a partial stack of regular units in peace time, and then recruiting mercs to ramp up to a full stack when needed.)
The Samnites are an interesting faction as they get bonuses in field battles based on terrain if the army is in normal stance. While they can't use them in sieges, still can be handy and add some nice decision making as you try to make the most of terrain. Their Government Action to summon a full stack Ver Sacrum army is very nice, and can be used both offensively and defensively (being able to summon an army to your capital in the nick of time to save it saved my bacon more than once.) There are also techs that make the Ver Sacrum army better, meaning it's still useful even later in the campaign.
Most recently I've played the Ventii, who are cavalry focused. Not my usual type of faction, but I had a lot of fun using their lighting fast cavalry to flank and harass, while their infantry held the opponent in place. Kind of reminded me of my Masaesyli campaign. Their Government Action to decide what kinds of horses to breed for the coming year is also very nice, with interesting tradeoffs between choosing horses for war or for work. They also have a nice sideline in Scouts, with a few techs to boost your Spies.
As for politics, I suggest
my post in this thread as well as
this thread for some general tips.
Hope that helps.
All the Best,
Welsh Dragon.