It might be a bug that while importing your photos to the iCloud library, many fail to import for no apparent reason, and that when you try re-importing again end up with thousands of duplicates.
It might be a bug that all, web, desktop Photos.app, and mobile Photos.app get almost--to fully--unusable with a 120 GB library size.
It is not a bug that there is no selective synchronization, and/or one way synchronization, so I can synchronize photos from my mobile devices to iCloud without pulling down existing photos on iCloud to my mobile device.
I agree Photos.app in all devices, and the iCloud is not really viable unless you are a light user. This guy [1] has similar complains.
Failing imports / duplicates is another issue I've heard about. I haven't personally experienced it and don't know how prevalent it is, but I hope they fix it soon. I don't know about library size - I've heard of people with massive libraries using Photos.app without trouble, but perhaps they had a Mac Pro or something.
As for selective synchronisation, I'd say that it won't be needed if they fix the issues (mentioned above) with library size optimisation. The point is: the user shouldn't have to worry about filling up their devices with photos. As soon as they're running low on space, Photos will reduce the library size. Now, that might not be working perfectly at the moment (not sure, works ok for me), but again - I'd call that a bug.
As I said earlier, the main problem here is whether Apple keeps working on this and gets updates out quickly. The worst problem of all would be if we don't see another Photos update until the next version of the OS (well ok, that's only a month or two away, but nevertheless). If Apple iterates quickly and keeps on improving the service, personally I'll be happy.