Answer depends on what tasks are being done.
Some workflows are very easy to move to iPhone/iPad right now. Drawing diagrams or marking up PDFs is much easier on iPad. Remote Desktop is the same on both, but more convenient on an LTE iPad than on most any other device I can think of. Writing is probably the same. Web usage feels about the same. Productivity (Pages/Numbers/Keynote or Word/Excel/PowerPoint) feels the same.
I can't comment for Audio or Video workflows.
I moved my .Net development environment to a server at work, and have been working off a Raspberry Pi 3 connected to WiFi, a 2560x1440 screen and a TextBlade and a Microsoft Sculpt Bluetooth mouse, and this has been my arrangement for months.
I use an iPad Pro 2 when I am out of the office, or when I want to work at a different spot in the office.
Whereas in the past I would prefer a MacBook Pro/PC laptop/PC desktop, but the past few months have shown me I don't need a laptop or desktop. With VMs for rent (like Azure or Amazon EC) or repurposing old server hardware, or a powerful desktop, I don't need to have a PC or Mac with me. It's actually more value having all our test DBs online and tons of storage, while paying very little and carrying even less!
The new A11 chip in the iPhone 8/8 Plus/X is faster that a lot of laptops and even certain MacBook Pro configurations (for certain professor benchmarks). Those of us using older hardware would get more computation from a mobile device that draws a fraction of the energy, and whose performance improves by 30-70% year over year. Meanwhile Intel is improving performance in the single to low-double-digit percentage in the same timeframe.
I think that's why Apple said that Post-PC devices will become dominant. Having a great keyboard to use with them makes all the difference. I wonder if they will port things like Jump Desktop to AppleTV, for those of us who want large, high-res displays, but don't need an iMac behind them.