I wonder how much of that is just out of convenience. Most people don't have a super detailed background on everything they cover. They'll get a thing, read over the marketing bullet points, and then share some of the basics.
I think it's a common fear that if you share bad reviews, people will stop sending you stuff. I'm not sure that's true in my experience. If you're just being mean for no reason, sure people aren't gonna wanna work with you. If you're being up front and fair, I've seen companies get excited about sending new models afterwards because they're looking forward to seeing how you like the improvements they've implemented based on earlier feedback.
Getting free review units is certainly nice, but we're talking about dashcams that cost a couple hundred dollars here and can easily be ordered online. It's not like a car where it's unrealistic for most everyone to just go out and buy every car they want to review. If you really wanna test a dashcam and a company won't send you one for whatever reason, you can go buy one for yourself or you can hop on a forum like this and ask if anyone has one they'll be willing to lend to you.
Having done YouTube reviews myself years and years ago, and in regular contact with some of the bigger YouTubers myself (who started in the days when we were in contact together with Nokia PR, circa-2014/2015 etc), this is
very much the case. They won't go publicly on record with it so as to not damage their reputation, but in working with brands this has been a consistent experience with them. They may have the odd brand that takes on positive feedback as you say, but a lot of brands also do not as they wish for positive feedback to sell their product, as
@Nigel said. To be honest, I probably would have gone all-in as a YouTuber in those years but I had just started a family, so my workings with Nokia and just my tech obsession took a back seat for a few years, by the time I got time to myself again, YouTube had already started blowing up haha.
You make the point about dash cameras that cost a couple of hundred dollars - for lots of channels starting out, whether it's a $1000 device or a $200 device the consensus is going to be the same. They may wish to keep the review units for their own tech needs (future comparison videos) or tech obsessions, or they may wish to sell them on so as to buy review units if a company is not going to send it to them. There are very few YouTube channels that have objective reviews out there (and if so, a lot of them are big channels who are in the position to pick and choose who they work with), and the market has only become increasingly cut-throat in the years since, so every little channel has to work hard to get subscribers and grow their channel. It's easier for them to read the marketing points, and say perhaps one negative thing about the product, but their byline is to buy that device. And every other device as well lol. They're not going to flat-out not recommend buying a device, as to harm their chances with getting future review units.
The thing is, in the early days of YouTube, brands were excited to work with influencers and the like, to garner feedback both negative and positive for their product. The same applied to written blogs now (which I've seen in the last few years have followed the same structure as what is happening with YouTube), and the same when I worked with Nokia back in the day. Nowadays, there's a zillion phone companies coming out of China, and other tech devices too, and they just want the positive PR spun around their products. They live and die by what someone says online, with the thinning profit margins.
