SweetFX is in theory, no different from Steam Overlay or Overwolf Overlay. And both those are safe.
Fun Fact: If I use the Steam overlay with GW2 sometimes after closing the overlay the dodge button and 'H' shortcut for the Hero panel stop working. The cure is mashing the overlay key combo over and over until I can dodge again. So I don't bother running Steam if I'm playing GW2 because why bother if no overlay? (Yes yes borderless window, I know).
This is now my excuse for standing there and getting hit by everything. Good thing a rabid necro can do that. And all my alts.
EDIT: New page, have a picture of a giraffe: