I'm assuming you're talking about vlogging. Because if you're talking about a 'reaction' video, it pretty well can't be scripted.
For vlogging, I've done it both ways. I've settled on making point form notes before starting.
The problem I find with videos that are totally unscripted (just a topic in mind) is that:
- They are generally far too long.
- They aren't very funny or interesting.
- Most people lack the confidence to execute solid/entertaining delivery when they aren't confident about what to say next.
I know proper editing can fix some of the points I just made, but seriously... it's not that hard to make some point form notes so at least your video follows some kind of logical order and you delivery some key/interesting/funny lines.