I think any issues with abuse of strong upvotes is tempered by the fact that someone has to spend a lot of time writing posts and getting upvotes from other forum members before they can have much influence with their votes, strong or otherwise. So in practice this is probably not a problem, because the trust is earned through months and years of writing the posts and comments and getting the votes that earn one a lot of karma.
I think any issues with abuse of strong upvotes is tempered by the fact that someone has to spend a lot of time writing posts and getting upvotes from other forum members before they can have much influence with their votes, strong or otherwise. So in practice this is probably not a problem, because the trust is earned through months and years of writing the posts and comments and getting the votes that earn one a lot of karma.