Embargoes do get a bit of backlash sometimes, but not nearly enough.

Why should a full embargo get backlash? They are trying to get input for an understanding, controlled population before unleashing it on a wider public. The whole idea is that the preview is not representative enough to start setting expectations for everyone. But it is far enough along to get the general idea and get feedback to address.

I am constantly testing pretty well known products in advance of their release and they are frequently crap. Like one thing I'm working on hasn't been able to work at all for a week due to some bugs that something I did triggered and they haven't provided an update yet. However when they actually are available to the general customers, they are pretty much always solid and get good reviews. If I publicly reviewed it, it could tank this product even though no one could possibly hit most of the stuff that I hit.

A full embargo seems fair. The selective embargo seems like an unfair idea, but also is a bad idea. If everyone knows they are allowed to talk about it, but only the good parts, then people will be speculating on what is not said. One product I tested had someone fanboying so hard about it they were begging the product team to lift the embargo so they could share their enthusiasm. They said no, they didn't want partially informed internet speculation running until they could address all aspects of the product publicly, and frankly there was too much crappy parts even if he was over the moon over the product and didn't really use the bad parts.

I suppose I could see being uncomfortable with the "testers" also being the likely "reviewers", because your are developing to the tastes of specific reviewers and tailoring for a good review in the end even if those reviewers aren't fully representative of the general population. It's easier to get a few dozen key influencers happy by catering to them/making them feel special, than releasing a product and hoping you hit their sensibilities.