Could you live daily, only on your smartphone, if push came to shove? Or, you were literally stuck on an island with only it?
Web browsing it would be ok, but very frustrating. Especially, needing to pinch to zoom for many things. Also, having to use mobile friendly websites.
Videos, it would be alright, especially with my nearly 6 inch display. I could output videos to a second display through USB- C or Miracast.
Gaming: There are lot of fun, well rounded games like Fallout Shelter, Angry Birds, and even a ported Gran Theft Auto Vice City. You can use a controller to play them. However, gaming on a phone, is not nearly as great, as a laptop, desktop or console.
Word Processing, PowerPoint, and Excel: With a Bluetooth keyboard, it would be ok. It would definitely be a far cry from using my Windows laptop and desktop.
Video Editing and photo editing: There are simple photo and video editing apps, but are woefully inadequate. They are not nearly as feature rich or powerful as premiere or photoshop.
Other professional work like web design: Not possible, unless there was a powerful web design app, equalivilant to Dreamweaver.