From what I see on review thread I'll say the possibility of those users using  those platforms they review are far less than the percentage you gave them. Most of the reviews remarks are similar which just goes to show people are copying from what others above them have said to write theirs. I haven't participated in a review campaign and I don't think I'll do that in the future either unless it's a service I don't know how it works and what to try them out. Majority  of the review participate don't use the platform they review after the campaign is over, they're after the money the projects are paying.

Also I just feel review shouldn't be paid upfront, let the users use their funds for the review and those with authentic feedback get payback or payment should be based on your remarks and not a general payrate irrespective of how your feedback is. Review campaign does no impactive marketing for the project but just to get users feedback on how theirs platforms are.

Signature Campaign is a better promotional campaign than review campaign, the visibility in signature campaign is more than that of review campaign and the impact as well. Just as others have side both serve their different purpose so obviously both has their benefits but if you're looking to promote your project go for signature campaigns and if you're looking for a direct feedback on your platforms go for review campaign. Signature Campaign gives your project more exposure and you can also get free reviews from users that use them (both the participate of the campaign and others).