I've had an obsession with Bretonnia recently so I've played all the campaigns multiple times on difficulties up to very hard. They've always been pretty difficult in the beginning but definitely doable. I'm pretty sure they are near impossible without some extremely clever chain heroic victories against superior forces, or hilariously massive amounts of luck. Bordeleaux isn't "harder" but just has to deal with a full province vampire faction, meaning knights or any high leadership units are going to be super important.
Here's what's different in my experience and some problems:
I've only played 2 of the 3 factions since the patch on vh/vh and I've noticed less starting knights and no mobs. Awesome with the no mobs, scary with less knights.
Bretonnia: (pretty much unplayable) The Greenskin faction in the mountains are extremely aggressive compared to what I've seen before. They will not only attack you from the northern side of the mountains, but will also send sneaky forces to the south - taking your port from under you. Marienburg is probably about the same in aggression or more, but it stacks. If you're moderately lucky, you'll have some breathing room so you can deal with an incursion and only the greenskins or only Marienburg two at a time, but sometimes you might have to deal with all 3 in a single turn. To top it off, the red duke ALWAYS focuses on Lyonesse and ALWAYS gets it - it use to be more timid or focus on the stronger factions. So you could have a 4th in your little blood orgy party - hasn't happened yet, but I'm not looking forward to it. Lyonesse always being conquered also means 6 turn tech is effectively 6 turns for a small chivalry boost.
Bordealux: (Not unplayable) Not too much different minus the lack of a cavalry unit (I'm positive it started with 2 knights of the realm before). Again the tech tree is stupid simply because you HAVE to get a 6 turn tech that's effectively 10 chivalry (Lyonesse will never be around to absorb). The incursions are fine, and it's nice to not have Mousillon sneaking around trying to find openings to attack your province, but now, you will always have to deal with a much stronger vampire force when you do face them as they will now have a full province. I've been just chain restarting to see if I get a game where Lyonesse doesn't get conquered, but it seems you have to really rush the cavalry buildings and hope they finish before the Red Duke comes.
I know it's Very Hard, and I know you'll say "play on a lower difficulty" or "why are you playing Bretonnia?" but I like Bretonnia and I've played multiple campaigns on Very Hard on Bretonnia before. The balances just made them possibly impossible or just way harder. I'm sure I'll love those cavalry buffs if I ever get to have more than 2 cavalry units