Only that you have to rest 5 minutes each time you use it.
Quoting my post from another thread:
My party often tries to do this (to the extent that I'd considered removing the roll from the 'word' powers to make them static/predictable), but it has bitten them at least twice and they've also chosen not to do it at least twice:
Let's say the party finishes a major battle with the following hp totals:
25/40
15/30
4/30
And completely out of any healing powers.
If they rest 5 minutes and don't spend any healing surges, they get to spend their 2 words for surge+1d6+5 say, so now they're 25/40, 30/30, 19/30... they go to rest again, and they're attacked.
In that battle, they're down hp (a hit from bloodied, on two people) _and_ they don't have their healing words. That's a real penalty.