For me personally it comes down to the robustness of the apps available on iPad. I spend a lot of time in my day job in Excel, and the iPad version both lacks certain advanced features, but it is overall clunkier to manipulate. Excel in general I find works better with a mouse, not even a trackpad, for how I use it, and Microsoft’s mouse support for its iPad Office apps is half-baked at best.
For most other deficiencies I find that it’s due to third party developers not embracing the latest technologies, or even ones that are a few years old. Still not all apps have proper mouse support. Many apps barely support the Files interface. Most of what I could do on a desktop can be done on an iPad but it requires extra clicks or other workarounds. I was encouraged by the fact that Apple mentioned desktop-class apps for iPad during their presentation, not only cleaning up and refreshing their own stock apps but also adding APIs and other resources to allow developers to do the same. The onus is on developers to actually use them.