Can't really use Rylai as an example as its an item. But even if you do, a slow and a complete fear/stun are different, one is a hinderance, one is hard CC even if its short.
Ranged also can't be used since that doesn't mean anything. They are ranged and he isn't. That is all there is to it. That could be said from any ranged champ and any melee champ, still doesn't mean anything nor will it ever mean anything.
And maybe Shaco isn't meant to effectively use FQC? Like is that surprising that not all items are best for certain champs? He isn't a support, so of course he won't make the best use of it.
And going off that point and your last question. You basically ignored the part where Shaco's boxes are 1. Invisible. B. Untargetable until they activate. C. Hard CC when activated. Regardless of ranged or item slows or not, you can see Zyra's melee or ranged plants, you can see Heimer's turrets and roughly know their ranges, you can't see Shaco's boxes and if you activate them, the is hard cc that will no matter what hit you 1-3 times without question. So how you can compare the 2 is truly baffling to me.
If this is about the gold it gives from hitting, then just get Ancient coin if you just want to hide in bush all day long. That does the same job as gold is concerned and isn't too strong on Shaco.