Generally, the video tag is great and has come a very long way from when Video.js was first created. If the way you think about video is basically an image with a play button, then the video tag works well. If at some point you need Video.js, it'll become obvious pretty quick. Notable differences include:

* Consistent, stylable controls across browsers (browsers each change their native controls over time)

* Advanced features like analytics, ABR, ads, DRM, 360 video (not all of those are in the new version yet)

* Configurable features (with browsers UIs you mostly get what you get)

* A common API to many streaming formats (mp4/mp3, HLS, DASH) and services (Youtube, Vimeo, Wistia)

Of course many of those things are doable with the video tag itself, because (aside from the iframe players) video.js uses the video tag under the hood. But to add those features you're going to end up building something like video.js.