True, but in my experience more so on iOS than on Android/Windows 8 Tablet -- though Windows 8 isn't fair since it's a full OS.
I mean, what are we saying when we say productive? As in, do my day-to-day job on it? Hells no. The only tablet I could do that on would be the Surface Pro, and at that point, I'll just use a laptop since that's what it is anyway. Also, almost my entire job is Windows dependent. The webapp we use is IE only for crying out loud (it sucks, I hate it).
But there are people that do their job on the iPad, and run businesses off of it and their iPhone, and don't seem to be hating themselves, so it is possible. It just isn't everything to everyone, but I don't think it is supposed to be.
At home I use mine for almost everything I was using my laptop for, and it is more convenient than even my Air, and has MUCH better battery life.