If you want a challenge, try the British. While having a nearly invulnerable island production machine might sound nice, the British have two big problems: 1) They have no cavalry (all their cav units are chariots), 2) they have no archers (just slingers).

Usually I can deal with no cav or no archers, but not both. The problem is when facing an enemy who fields archers. Counter archer fire can solve that problem, as can cavalry, but with neither, you're pretty much resigned to being a pincushion. Fortunatly that doesn't matter during the early part of the campaign (the other barbarian factions don't have much in the way of missile troops either). But if the Roman troops start fielding archers, or you head east and run into the Parthians, you are going to be in big trouble.

The chariots suffer from a big problem in that they can't "mop up" very easily. While they can cause units to rout effectively, they cannot take advantage of that rout like cavalry can. They simply do not kill spread-out units quickly enough. I've seen some British chariot units spend 10 time-accelerated minutes trying to kill the last dozen or so men in a routing unit before. A simple unit of light cavalry can kill hundreds of fleeing enemy men in the same time.

Oh, and if you like playing on hard or worse battlefield AI, you will have to deal with the attack/defensive bonuses the AI gets in hand-to-hand combat. I did a bunch of tests, and on Hard, the AI gets a huge combat bonus (in controlled Hastati vs Hastati tests I saw the AI beat me 100% of the time with consistently 2/3 of their men left and about 5 of my men left). What this means is that you have to use flanking tactics to win, and w/o cavalry, this becomes a lot harder. Head-on fights simply won't work on Hard.

Thus, if anyone is able to win the big campaign (50 territories + rome) as the British, I'm going to be very impressed, even more impressed if they do it on Hard.