It's just got a specific niche is all. You need to be able to heal or shield your team, preferably with a low CD and AoE, and you need more than just your ADC that can benefit from bonus AS.
If you buy this and it can't fulfill its purpose, you shouldn't have bought it. If you buy it and each of these requirements are met, then this is an amazing item. It's all about itemizing according to what is needed (nobody would say Archangel's Staff is a bad item, but it is if you buy it on a champ that will never be able to max stacks and make Seraph's Embrace).
Try using it with Sona when you have multiple ADC's like Tristana mid. Use it in the correct way and you are rewarded. Use it just because you feel like making it and you'll be punished.