I was a Notability user but having to add the .png files (stickers,clip art) to my photo library before I could access them in Notability was a big pain. With GoodNotes I organize all of my stickers in directories and just use the elements tool to pull them. So much cleaner. Notability locks down their “Stickers” menu to just the in-app purchases.
That said, if Notability added this capability, I would probably return because the Notability with its organization, is a the superior app overall.