For propaganda, this will be great. The thing is, you don't even need convincing videos for propaganda to work.
For truly scary things, like falsifying evidence, I think it will be awhile before this will get past expert analysis, or even a group of people on Reddit trying to prove it wrong.
In the long term, video will simply be treated like photos are now. With disbelief.
It can also be used to 'ratfuck', if a video is released with a politician saying a gaffe, they can release a bunch of similar but fake videos and then claim that 'all of them' are fake. The confusion would sow doubt- the technique could be used offensively as well as defensively.
For truly scary things, like falsifying evidence, I think it will be awhile before this will get past expert analysis, or even a group of people on Reddit trying to prove it wrong.
In the long term, video will simply be treated like photos are now. With disbelief.