I still think it's less powerful than encounter powers, and even twin strike. The possibility of critting, are low enough and random enough that you can't count on them for big damage when you need it.
Twin Strike is arguably one of the best damage Striker powers in the game when combined with Hunter's Quarry.
Even so, at level 11 (shy of other feats and abilities) with a 22 stat (like your PC has) and a +3 magic weapon, damage over 20 rounds for average same level foes is:
TS Melee Longsword: +17 vs AC 25, D8+3+6+7, first attack = 7 misses, 12 hits (20.5 per), 1 critical (34.5 per), + second attack = 7/20 * (7 misses, 12 hits (20.5 per), 1 critical (34.5 per)) + 13/20 * (7 misses, 12 hits (13.5 per), 1 critical (27.5 per)) = 502 damage, ~25 per round
TS Melee Longbow: +16 vs AC 25, D10+3+6+7, first attack = 8 misses, 11 hits (21.5 per), 1 critical (36.5 per), + second attack = 8/20 * (8 misses, 11 hits (21.5 per), 1 critical (36.5 per)) + 12/20 * (8 misses, 11 hits (14.5 per), 1 critical (29.5 per)) = 496 damage, ~25 per round
Hand of Radiance: +14 vs Reflex 23, D4+3+6, each attack = 8 misses, 11 hits (11.5 per), 1 critical (23.5 per) = 450 damage, ~22.5 per round
I consider 90% of vanilla Twin Strike plus Hunter's Quarry damage to be pretty darn powerful. Striker level powerful, not Controller level powerful. And unlike Twin Strike plus Hunter's Quarry, this is without any class features. This is straight up the At Will power plus magic weapon and nothing else.
Sure, Twin Strike can be used against one foe and hence is more focused fire. Sure, Twin Strike can be beefed up a lot with feats. Sure, Twin Strike is still great against solos (that's why the game system has two+ At Wills per PC, not one). On the other hand, Hand of Radiance is not limited to the nearest enemy like Hunter's Quarry, does radiant damage, and can kill 3 minions in a round instead of 2.
But to even have a non-Striker At Will in the same ballpark as the generally considered borderline broken Twin Strike plus Hunter's Quarry is to consider that At Will auto-borderline broken as well.
Note: At level 10, Hand of Radiance does slightly more damage than vanilla Twin Strike plus Hunter's Quarry. TS + HQ gets the 11th level HQ boost to surge ahead.
The fact that you need to pull Twin Strike into the conversation to justify a "it's not too powerful" POV indicates a potential problem with Hand of Radiance.
Another side effect of counting on spike damage is that it often will show up when you don't need it. A baddie down to 10 hp getting crit wastes a lot of damage.
This is going to happen when the At Wills are pulled out anyway, regardless of the type of At Will attacks being used. Many of the Encounters and Dailies that are going to be cast in the encounter have been, so players start using At Wills more. This occurs typically near the mid-point to end of the battle where foes are often blooded and hence, the odds of this occurring are greater.