Sure. I would guess they are worried about their reputation towards manufacturers if they bash their product ..
No, that is not the reason (at least not in film photography). It is more about getting the widest range / coverage at the lowest costs. Introducing beginners to your brand. And that is were instagram currently has the advantage.
I'm guessing also that most of the vloggers are just getting stuff for free.
No, not in general (and especially not in film photography as a general rule). It depends very much on the industrial sector, the specific product and the importance of your channel (number of followers).
And what you as a vlogger are doing: Introduction to a product, just advertizing or a really detailed product review.
Example:
You have a small(er) film photography channel and you are doing detailed film reviews. Lets assume you get 10 films for free from a supplier. Then you would have to do the test work for several days (it takes so much time if you want a really proper, well-done review), and then you have to do the work to edit your 10-15 film on youtube. The creation of such a youtube video needs also quite a lot of work. It cannot be done in an hour.
If you look at all the work and the opportunity costs you have for these days (in that time you cannot go to your normal job earning money) you immediately see that you are loosing lots of money. Some films given for free cannot compensate that at all! It doesn't change the situation. From an economic point of view such detailed reviews are a loosing deal for you.
Different example:
I know that some of the new Chinese lens manufacturers are quite active in influencer marketing: As their brand names are quite unknown compared to the established brands they know they have to make the potential customer base more aware of them. They send out lenses to popular vloggers / influencers and say them "costs of sending back may be too high, you can keep the lens"

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Therefore I am a bit cautious concerning some published "reviews" of such lenses

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But back to the ADOX SCALA kit:
My test results:
- wonderful quality with the ADOX SCALA films
- very easy handling
- a huge step forward concerning terms of being environmentally and user/health friendly
- low costs.
For me as a BW transparency lover and user of the original Agfa Scala process a dream has come true.
Best regards,
Henning