No, the problem is Seal of Binding. You don't necessarily need Demigod to make it work, it's just the easiest way. You can repeat the trick with any source of healing or regeneration that will allow you to heal an adequate number of hit points (and the number of ways to do this will grow as time passes and splatbooks spread like bloated, power-ridden tumours).
The damage inflicted by the power itself is not really its selling point. It's the guaranteed 50 rounds of inactivity on the part of your enemy which makes it sing. During this time, other PCs can lay on the hurtin' with non-attack damaging effects.
The attack roll to make it work also becomes a triviality at the level which you will gain it at: an elven divine oracle can roll 3 times to see if he hits! And he will probably have many bonuses supplied by other party members.
Orcus's Aura is relevant, but there are trivial ways to defeat this problem, foremost among them the brilliant and under-appreciated Wizard utility power, Resistance.
What makes the power badly designed is that the Rules As Written do not match the Rules As Intended. As written, you can affect the target with additional damage. As written, you can bypass the built-in limiter on the power and make it last for five minutes with sufficient healing. I'm almost certain this was not intended.
And unlike most other examples of this in the 4E rulebook, this will have a bad effect on game play as well, turning what should be a dramatic battle into an exceptionally dull one. It is a badly designed power, and it needs errata. GMs need to houserule it out of the box, if their players spot and use this trick. Shoudn't have made it past playtesting.
(EDIT: Just figured the following. With a power that grants regeneration x, you will get an average number of rounds of stunlock equal to 269 / (20 - x). This assumes a Cleric with a Con of 10, Wis of 28, and no additional healing except his own healing words (so no paladins, warlords or healing potions). I have also not figured the Wisdom modifier for Healing Lore into the regen figure, because this will be on a case-by-case basis.
With a regen of 10 (trivial, holy wrath lvl 19 Cleric Daily exploit), you get 26 rounds of stunlock and damage. With a regen of 15 (which you could get from holy wrath dependent on the GMs decision on how Healing Lore interacts with regeneration powers), you get the full 50 rounds. Add in healing from other sources, and your number of rounds will also increase (by an amount equal to the [amount of healing] / (20 - x)).
Even with a regen of 10 attainable by a non-demigod, this power is still far too effective and far too dull, and badly designed for all the reasons outlined above.)