The truth is that the whole PR thing is mutually beneficial, which means that both parties give something into the relationship and both parties get something out of the relationship as well. The Corporation gives the product, the you tuber gives a review. And the review can be either positive, negative or neutral. That's a given. Now how a review is presented, in my opinion, is where gratitude part comes in. It should be an imperative giving a negative review politely and honestly. Seriously, is it really necessary to say that something total failure / crap / the worst thing ever?? No... Because, just because something did not work for you does not mean it will not work for me.... I do not know about you people, but when I watch a review I am trying to figure out whether or not a particular product could be a good fit for me personally, which is why I tend to watch you tubers that really try to give a very comprehensive review of a product that is not based on their own personal experience after only one or two uses (and it's so easy to see who's done the work and who's just featuring the stuff). Long story short, gratitude is expressed in the way you talk about the product and the way you treat the product in a video.... it does not need to be spoken about, it needs to be shown. And really good you tubers who run their channel like a business know that - they need to nurture the relationships with both the Corporations and the Viewers and they need to do it politely and honestly.

Not quite sure if I've explained my point of view well, but I hop I did.