I agree that many of those items (harassment, copyright-infringing material, self-promotion, offensive language) are vague and could be abused by Vivaldi if they were out to get you for whatever reason. So it comes down to whether or not you trust Vivaldi to enforce their policy reasonably and fairly. If you don't, then you shouldn't use them.
At the end of the day, any service provider needs to have some discretion on these things, because the line between malicious and legitimate usage can be too blurry to express as a precisely defined policy.