I've been playing MTW long enough to have taken every faction in every era to victory - though some are much harder than others

In VI, I always play for the 110% victory, and take over the Viking lands as well, by intense use of agents. Getting those two provinces can be almost as much struggle as conquering the whole of Britain, what with rebellions and re-emergences and no connection to your king... but it is fun. It does mean you have to leave one British province unconquered until later so the game doesn't end.

In VI I find the Welsh and Northumbrians hardest, but they are both doable. As others have noted, the Vikings are VERY loyal allies, so the first thing is to get that alliance ASAP, throw every available bishop, emissary or preferably princess at them and hope they get the deal done before the Vikings spot Lindisfarne, or get into the Irish Sea, as appropriate.

In Northumbrian campaigns, you need to keep a big stack in Elmet, and SMALL armies in the outer provinces - I've tested this many times - the AI will more often attack a sizable army than a small one (try it sometime - save a year when your one of the 'outer' provinces is attacked, then reload it and reduce the garrison - no attack....)

By having a big stack in Elmet you can always retreat to stronghold in the outer ones, then counterattack next turn. It takes a while but Elmet has iron, so you can eventually tech up nicely - let's face it, you aren't going to waste time and money building farms there.... though this does mean you'll have to get your cavalry elsewhere.

With the Welsh, it's a touch and go turtle strategy - make nice with the neighbours as long as you can. Unlike the Vikings, the Mercians can't be trusted, but if you can hold out until they and the Saxons go at each other, one or other of them will finally erupt into civil war, and it's your job to seize the opportunity created. The Welsh have iron - lots of it - and good missile options. The alternative is to go for the Irish first, before they come your way.

In the Mediaeval campaign, I think my least favourite is HRE - classic overstretch starting position, surrounded by chancers who like the look of your territory...