Adam C. Engst at TidBits:
Luckily, unlike many people, I don’t have a data cap for my Time Warner Internet connection, so at least that wasn’t a problem for me, as it might be for you. Where I did run into trouble is with my iPhone, on which I’m using the Optimize iPhone Storage option to reduce the amount of data transferred and stored. Tonya, Tristan, and I now share 2 GB of data on our family plan, and before last month, we had never come close to using that much, since we’re still accustomed to having only 250 MB each. So you can imagine my surprise shortly after I enabled iCloud Photo Library when AT&T texted me to say that I was approaching my 2 GB limit. […]
There are no settings to prevent iCloud Photo Library from working over cellular, and while I disabled cellular data for the Photos app, that made no difference. I could turn off cellular data in general (and I did once or twice, but that’s a hard thing to remember every time you leave the house), but by the end of the billing period, AT&T had hit me with $30 of overage charges for two $15 blocks of 1 GB of additional data.
It doesn’t appear as though full photos are being uploaded — that should only happen over WiFi, or at least that’s what happens for me. But there should be more stringent limitations baked into iOS for how much cellular data any app may transfer. Even though LTE has the bandwidth to support broadband-level services, its users often don’t have broadband-level data caps.