After playing with and upgrading this card, I've really grown to hate it. Not because it's a bad card (it's great, honestly) but because it's so flexible that it will stifle competitive design for other Guardian weapons, especially the low level ones.
Think of it this way: for level 1, its "competition" is The Hungering Blade, which is already...not great. But let's think about future designs, and review what they have to compete with.
With 1 XP invested, Runic Axe is a 4cost 2-handed weapon with 4 charges, recharging one charge per round (and can still attack for +1 Fight with zero charges, giving it a flexibility edge over anything that needs charges or ammo). In addition, Runic Axe will have either
Spend 1 Charge: Get +2 Fight on attack
Spend 1 Charge: Deal +1 damage on attack
Is a Relic and only costs 3 instead of 4 (nice for Guardians who often struggle to scrounge cash)
OR
Spend 1 Charge: Get +2 Fight on attack
Spend 1 Charge: Deal +1 damage on attack
Spend 1 Charge: If this attack defeats an enemy, either heal 1 horror, heal 1 damage, or draw 1 card
OR
Spend 1 Charge: Get +2 Fight on attack
Spend 1 Charge: Deal +1 damage on attack
Spend 1 Charge: If this attack succeeds by the location's shroud or greater, discover 1 clue at location
OR
Spend 1 Charge: Get +2 Fight on attack
Spend 1 Charge: Deal +1 damage on attack
Spend 1 Charge: Get a free move (that doesn't trigger AoO) or a free engage
OR
Spend 1 Charge: Get +2 Fight on attack
Spend 1 Charge: Deal +1 damage on attack
Spend 1 Charge: If this attack is successful, deal 1 damage to each other enemy engaged with you
Any future 1 XP Guardian weapon has to compete with that, and compete with the fact that you get 2 copies of it for 1 XP, and compete with the growth over time, and compete with the not-so-unlikely chance that you'll Refine it to make it stronger without even getting charged XP. That basically boils down to forcing "good" future 1 XP Guardian weapons to be one-handed, and even then, they'll feel extremely lackluster unless they come with either guaranteed bonus damage and some kind of reactive ability (for things with ammo), or conditional bonus damage that you can reliably build your deck around, unlike Hungering Blade and some kind of reactive ability (for things you don't have to reload). Whatever the bonus or condition is probably needs to be good in multiples, too, to make spending 2 XP on a pair of copies not feel worse than spending 1 XP on 2 copies of Axe.
Tl;dr: Runic Axe is great, impactful, and incredibly flexible. Everything that makes it a worthwhile inclusion as a card also forces power creep into future 1 or 2 XP Guardian weapons to be worth including in any deck. It's also hard to nerf, and the weakest thing about it is that it takes 2 hands.