My two cents.
In the end, every reviewer is getting paid, whether it working for a magazine or YouTube likes. I for one can’t remember ever seeing a truly bad review, I have seen some with some dislikes. I have also heard many reviewers say that they won’t do bad reviews and that they turn down some products and that all makes perfect sense to me.
So in conclusion, I feel that if they tell you upfront that they bought the item at a greatly reduced price or that they were outright given the piece, there shouldn’t be a problem.