The 3rd level power spike. Fire Rune + Stone Rune on top of Giants Might. Amounts to two amazing SOS' + Rage-like. The archetype is IMBA through tier 1 and 2, i.e. the levels that see the most play.
I in fact like Rune Knight, but just find it a bit over-tuned.
I don't know. All of those abilities are very good, but they're each like a good 1st or 2nd level spell that you can use 1/long rest. It's a really good nova if you're stacking them, but heavily dependent on how often the DM is letting you recharge.