So I too am wondering what is going on with iCloud drive sync support with odrive and I am a little perplexed when you say that Apple does not offer a "public API" to iCloud. (I presume you mean both or either iCloud app storage as well as iCloud Drive.) Being that iCloud has vast support from applications which use it for synchronizing settings, data files, and so forth by use of CloudKit. Also, Apple provides a full SDK fir iCloud Drive as well with XCode 8. Therefore, I am wondering what functionality odrive needs to make iCloud support of either happen? Granted, iCloud is undoubtedly complex and frankly end users stand a good chance of accidentally breaking things by trying to manually muck about with CloudKit data files, so I suspect odrive would be wise to provide some level of obfuscation to try to help prevent that from happening. But even so it doesn't make a lot of sense when you say that the support simply does not exist when it clearly does. Perhaps it is more complex than what other cloud providers offer. (For instance, if you support B2, I should think you can support anything considering how it handles large files- then again I haven't tested your large file support on B2 so I have no idea if odrive actually supports this.) Anyway, if there are other reasons hampering iCloud support that are "more technical", you are dealing with some technical people here, so you don't have to shy away from "telling it like it is." For Mac Users, iCloud support is core, and to provide some other omnibus access to this Apple "black box" would be a huge offer.
All things told, even if you were to ask for more money for "expanded features" like iCloud support, I think people would pay it. What odrive is attempting is a really tall order, and I feel like you guys simply are trying to take on the world and considering your price point I can see how you don't have the resources to take on such a daunting undertaking. What I am suggesting is that it is not that Apple doesn't offer the API, it's that it's a very big task and Apple is notorious for changing how they do things every other OS release. I'd be reticent to support it too.
Anyway, I hope to hear from you about this.