You can win with every faction - and I always go for the 110% win in VI - ie I also take the Viking lands with carfeul use of agents. I've done it with all the British factions, including the Welsh, though I think this is the hardest faction, a serious 'turtle and tech-up' scenario. You get iron and valoured-up archers, on the plus side. On the downside, nasty neighbours with huscarles... you probably have to play diplomacy more with the Welsh, and make maximum use of agents, too.

But that's a bit o/t - with regard to rebels, if you're struggling with loyalty problems, try shuffling your troops around so that you leave the most defensible provinces (eg ones with a guaranteed river map) with the lowest loyalty, so if the rebels do pop up, they are easily defeated. You can even use this to 'farm' rebels - ie keep 'em coming and take lots of money off them in forfeited lands etc, but that is a bit on the cheesy side.