I had a Vivoactive 3 before uograding to the Fenix 6X.
The Vivoactive 3 is good for recording your rides and data for up to about 5-6 hours (ish), and displaying the usual stats.
The last 20% of battery dies a lot faster the the first 20%, so I always had to make sure it was charged before exercise. Without recording exercise it lasted for a couple of days between charges.
The reasons I replaced it were:
1. I wanted to use it to display a route. Although the Vivoactive can do this, is doesn’t overlay it on a map, which can make it difficult to interpret, especially off road.
2. Using it for navigation drastically reduces the battery life to sub 2 hours
3. The touch screen is nice for operating the watch when your not using it outside for exercise, but can be a PITA during exercise, where rain, sweat or water (I also use it kayaking) interfere with it.
Whilst it is obviously more expensive, the Fenix is a much better device for what I need – no touch screen, bigger screen that displays proper maps, way better battery life (10 days without exercise, 6-7 days with multiple exercise periods. It also generates more data and has support for more sports (Vivoactive only does cyclng, not road or MTB). You can add apps for extra sports, but they don’t always integrate as well.