The former isn't really so bad, as to get them you have to have won four battles or raids by 5 or more as the British, by which time the Saxons are either fleeing across the North Sea like whipped curs or have also had about that many wins and have piles of Warrior Group reinforcements and skirmishers.
The latter case, as an alternative for the Gwŷr y Gogledd, is, I think, a little problematic if you're facing the Saxons (more so than with other forces). Shock Cavalry roll double the number of dice in combat (and the rule is 'calculate the number of dice including bonuses, then double it' per a clarification from Rich) and on top of that do double Shock. Lets do the maths:
- 4 Elite Shock Cavalry = 4 dice
- Champion = 2 dice
- Lord = 3 dice
- Hero of the Age = 3 more dice for the Lord (remember, it's in the British starting hand) + 1 for the dragon-suited card
- total = 13
- Doubled = 26
With no other dice, that's an average of 13 hits. Against Elites, that's an average of 2-3 kills and 8-10 shock (because Shock is doubled). With an Aggressive Charge or Artorius (which is not unlikely, and something the British player will be looking for and hanging on to), the cavalry are hitting on 3's and that suddenly becomes 20-21 hits, 3-4 kills, a metric boatload of shock (14-16) and busted Amphorae all over the shop.
Meanwhile the 12 Saxon Elites, Lord and Champion they are (in the best case for the Saxons, attacking) have 17 dice, for an average of 2-3 kills and 4-5 shock. To be fair, that is likely to break the cavalry, but that is their worst case opponent - two lots of Warriors and a Level 2 Noble will have 14 dice and probably not break them, although they will likely incur excess shock while vaporising the Warriors (4-5 kills now and 8-10 Shock). Against a single Group and Level 2 Noble in the Border Tower raid the charging cavalry would automatically capture (3:1 dice odds).
The tactic for defending against Shock Cavalry is to keep them at range and fill them full of missile weapons, so they have too much shock (any at all) to charge. The other forces in Raiders have 12 or 16 missile-armed troops in their core force, so the Gwŷr y Gogledd going against them do at least have some chance of being held at bay. The poor Saxons have 4.
I'm teetering on the edge of deciding they're overpowered for game balance, and the rule should perhaps either be double dice OR double Shock, not both. I'm also wondering if woods are too lenient in terms of movement restrictions for Shock Cavalry (charging knee-to-knee at speed in a wood?)