I have no experience with them, but I will say that natural handles are the best.
They will never create a handle material that's as good as hickory or ash.
No non steel handle will last forever and when a synthetic one does break it'll be harder to replace ( and can't be replaced on a fiskars )
I can't say for a fact, but the fiskars are said to use a softer steel. ( they're also known to split great so while the factory grind would probably hold up fine, who knows about a much finer one )
personally I can't stand an all steel or synthetic handle.
But since you've been using the estwings for a while and like them, I'd just stick with them ( or restore a vintage hatchet , which is the best option anyway )
If you can find a fiskars hatchet on sale you should probably try one out, because that's the only way you'll know weather or not you like them.