Think about it, if you gave a reviewer your product and they severely critcized it, would you send them more? Inherent bias is why Blade magazine reviews of maker supplied knives are virtually useless.
This is something I've thought a lot about, and something that's really tricky with review channels, particularly if you're trying to make the channel your 'full time job'. I've talked about this some in this video:
For what it's worth, balancing this tension between 'giving honest negative feedback', 'generating friction with makers', and 'doing more good than harm' is harder and harder to balance as the channel grows, even without review samples being on the table. This is a part of the reason that, as the channel has grown and as more and more reviewers have entered the space doing great honest work, I've been slowly shifting my focus towards being a 'gem hunter', highlighting the 'best of the best' products, where I can honestly say there are few faults and describe them clearly, and just selling (or sending back to makers with feedback) pieces which don't measure up.
It's nice because, as I have less and less time, it allows me to spend more time finding great stuff and less time slogging through 'meh' and objectively flawed gear, and because it allows me to still be completely honest and straightforward with viewers, while having less negative effects, particularly on smaller makers for whom an honest-but-negative review from a major channel would be really rough. Plus, from a purely selfish perspective, I love nothing more than finding something that's gonna bring people joy, and sharing that joy, so if I can spend more of my time doing that, that's a win.
Anyways, brownshoe, this is a really good point, and it's something to consider any time somebody discloses that they've gotten a review sample (or, concerningly, doesn't). 'Objectivity' isn't a thing, so considering the potential (and guaranteed) sources and natures of bias in any information source is a great idea.