The iPad is a tool. Would someone argue that a hammer is flawed because it only can be used in some ways, some professions and for some tasks? A laptop is a tool, a PC is a tool, a mainframe is a tool, a calculator is a tool and a saw is a tool. So what?
Of course, a PC might be a better tool for your workflow, your work, your results. But that might be different for someone else. Work with the tools that fits your needs best!
Why do we need to criticize one tool because it is not equal or better to another? Gruber is right in regards of discoverability and consistency of (multitasking) gestures on iPadOS. There is room for improvement. The lack of "professional" iPad software should be directed more towards the developers than Apple - why needs some iPad app to be crippled in comparison to its Mac counterpart? Valid concerns.
But calling the iPad a failure because it doesn't fit to your work, your workflow and your processes? Because it isn't the ultimate general purpose tool? I know authors still using a typewriter. I know some that use iPads because of the "single app interface" and less distraction. I know others using laptops (and complaining about distraction). All are doing "real" work (as many other professions mentioned in this thread) and all have their preferred way of working.
If a developer/power user is happy with his Mac/Win/Linux setup, why does s/he need to criticize the iPad for NOT being the right tool? Or are these folks just unhappy with their setups and desperately hoping Apple will give them a better (-toy-) tool for less bucks? Or is it just en vogue to bash Apple?