I'm not really a huge Garnacho fan, but saying he can't do anything and was horrible is a little much.
He was a pretty good dribbler, but in a very direct way, almost abrupt. I'm tempted to say he's not a creative winger, but a lot of top sides tend to have a creative winger on one side and one of those direct ones on the other. Sometimes those roles are changed a bit or spread differently throughout the squad altogether, but there are players who were more skills and persistence than creative and ended up remembered as super stars, and I think a lot of the CR7 vs Messi debates show a personal preference there even. Plus the creative guy doesn't have to be the best one of the two.
Garnacho didn't need to be the most creative player, but if he's part of a successful side, he'll shine and finish off more chances (or make a lot of assists), but he's also simultaneously a little less dependent on the team to get any numbers on his sheet, since by his very style and nature just keeps on trying until it eventually just has to turn a little lucky. It can also kill of the rest of the team's chance at developing further and get the most out of their own attributes, though, so it's not always your best bet to get your team back into better times.
I do think Amad is more talented, but when he's not in form he turns a bit more invisible than Garnacho would. A playmaker who can't create loses most of who he is. It helps that he is also a pretty good finisher/good at direct play, but you do notice that he's a creator at heart. Occasionally he can "do a job", but I think on the long term I think he'd stagnate if all he gets to do is be a classical winger or hard working wingback, or even just a (non-false) striker. But if you find a balance between fitting him into a role with discipline and letting him loose a bit to add his personal touch and ideas to a game, he could become something special. If the balance in the squad is there, this could be at RWB, perhaps in a 4-2-3-1 he'd have to move to 10 or play to the side of a "finishing" 10, but if you manage to fit him in somewhere without chaining him to very specific responsibilities, he might just become a star if he adds enough consistency to his play.
But I think Amad will need some type of outlet on the other wing (and in striker and/or midfield), and then someone like Garnacho (but doesn't have to be him) can work wonders, not as competition for his place, but as an asset to get more out of a player like Amad.