I believe that species of aliens that are like the Grays or like the Nordics may exist, and I believe that hybrids may be possible...but I DON'T back Jacobs's integration/infiltration theory for several reasons.
I don't back Jacobs's theory partly because he thinks that all Nordic-like aliens are really Gray hybrids, and he doesn't believe in any benevolent aliens...which is a conclusion without any solid evidence backing it up. I disagree with that theory because there is a strong possibility that some human-like "Nordic" alien races that may exist are actually unrelated to the Grays and unaffiliated with them. Jacobs seems to think that there are only Grays, Insectoids and Hybrids, and that ALL of them are bad.
It's a logical fallacy to think there are only bad aliens out there and that there are only Grays, Insectoids and Hybrid aliens in the universe if there are any aliens at all, when the universe could in fact be full of thousands of millions of intelligent alien races.
In fact, in a universe as big as this one, some of the extrasolar species that could exist may be just like the so-called "Nordic" Space Brothers, and be aliens that look and act just like us without having to be Gray/human hybrids. Jacobs doesn't even consider that possibility.
Another reason why I don't back Jacobs's theory is because of the unethical and unscientific methods he used to reach that theory, and because he doesn't allow for any alternative explanations (like the psychosocial hypothesis) to be explored in explaining the abduction phenomenon.