If [sponsor X] has done five high-dollar 'product showcase' videos in the past year contributing [Y] in revenue, and a separate review of [sponsor X]'s product is coming up, does the writer and/or editorial staff have any possible filters (whether or not intended) to protect the revenue stream for future 'product showcase' videos from that sponsor?

I tend to give LTT the benefit of the doubt, but that's why I say it's a tightrope. Once you have paid content (in any form), and not just ad spots, it complicates the relationships you have with your audience, and with your sponsors/advertisers.