Concerns on Custom Skins

So I know that you are not allowed any custom skins that create an unfair advantage such as certain particle effects and the sizing on skill shot hit boxes such as maybe say Lux, but someone just told me that only the player with the custom skin can see the custom skin. Everyone will just see the default skin. So what's wrong with the particle effects and stuff if only YOU can see it. Maybe I'm missing the point here?. And a post also said that we were allowed to use custom skins for custom skins not to play with skins that you don't have such as rare skins like that twisted fate one. (Using custom skins that mock or are actually paid skins) Can riot see this or not because that dawnbringer Riven skin is pretty sexy I must say. Haven't tried using it though, and I won't until I get these question cleared out. For now I'm just using Pool party Riven and I've had yet to play normals. I've played bots and that's where they told me only I could see my skin :(. Anyways, I've written an essay, but that's all I have. Thanks