outside of the base stats on the item
the active is wasted
like buying shojin on nasus vayne or illaoi
you already have an in built shojin with your ult
buying the item itself wastes the active for which the item is bought for
Rakan guarantees ult hitting with his knockup and as the target is being cc'd heavily the slow from the glory is wasted
you can make the argument that you can use it after to continue the slow or to increase the speed of your initiation but there are better items that provide way more value to rakan specifically
ie that item is bad on him
this is further proven by the fact that if you go to rakans opgg page >items you will not find Glory built at all