For some reason the original post was deleted so I will repost it here:
A developer stated this in AMA
and the relative thread can be found here https://www.arcgames.com/en/forums/neverwinter#/discussion/1249443/critical-healing-redit-ama
From the small test that you will see below too, it seems that indeed Power affects the Critical chance of a heal, strange as it may sound. Specifically the greater the power value is over the critical value , the less Critical Heal % one has and if Power is lower than Crit than you can actually get the up to 50% chance.
The test was done in Stronghold with the help of the Piranhas. I dived it and healed myself 40 to 50 times to have a decent sample size, using the same power or at-will.
Here, with 193k power and 72k Crit I had 10% Critical Heals.
I then, dropped the Power to 62k and Critical was left at 70k. And tho I should have 2% less, my Crit heals actually increased to 39%.
As a 3rd stage I dropped my power to increase the gap of power to Crit even more and....
...the Critical heal% increased to 42%.
So yes, the Dev was actually true to his statement and the difference of Power to Crit actually affects the Critical Heal %. Think of it as the counter stat of Critical is Power when it comes to heals but will test it more on the days to come.
I have to say that this affects ONLY Critical Heals and it affects all 3 healing classes.
Hope it helps.