I'm an Information Technology manager and do much of my work on iOS (iPhone X and iPad Pro).
Most of our data is in the cloud (Office 365, Jira, Canvas, etc.) which makes it very easy to access from any device. The Office 365 apps for iOS are excellent and I often use OneNote, Word, and Excel in meetings. I also use our communications and video conferencing tools on both my iPhone and iPad: Slack, Skype for Business, Zoom.
Generally I use the iPad when I'm in a meeting or when it's handy. I use my iPhone when I'm more mobile, and I use my Apple Watch to glance at information. I have a Windows desktop at work that I use when I am at my desk and need the large displays and multi-tasking capability. At home I have a Windows desktop computer that I mostly use for photography.
There isn't much that I can't do on my iPhone or my iPad, and for those few things I can always remote into my desktop computer. It just comes down to comfort and convenience. I can work for quite a long time on the iPad, especially with a keyboard... and when I travel I don't bring anything else but my iOS devices.
These are incredibly capable devices, and so many of my tasks have migrated from desktop to iPhone and iPad. I manage my tasks, shared projects with my wife, and paying bills all from iOS. Then there is health/fitness stuff, maps, etc. My wife and I have also been reading our local newspaper on our iPads for years now.