Giaccherini is 28, he's not going to get better. Juve know who he is and they don't need him. Santon wasn't getting games for Inter. How are these two in any way similar to Niang? If you want to bring up examples then at least make sure they're comparable.
Italy international who is at one of the worlds best clubs, I'm fairly sure he would have had other clubs interested in him bigger than Sunderland? It's comparable in its equally unlikely. Ditto santon, had a rough patch but featured a lot for them before that would have had bigger clubs looking at him.
He's nothing amazing, but he'd defintley improve them.