Pallet wood in my area, and I have loaded and handled alot, has good hardwood. Most of what I have seen, is of Ash, Hickory, and Sycamore.
Sycamore is not a favored wood as most know! I have seen and saved some Walnut at times.
I would plug the nail holes and go for it. If anything you should be able to make up some kids bows!
Better than the chipper mill or worse!