FeNis can be extremely nice people, but they are also quite charismatic and inspirational in ways that Fi types are not. While Fi, in my opinion, takes a more distanced approach to relationships, Fe approaches these in a very dynamic, power-packed way. How many of these dictators were dynamic, inspiring men? Also, in looking to type a person on Ni, you can look at how abstract their speech is. If you look at the kind of comedian like Mitch Hedberg, that's the kind of linguistic abstraction I'm talking about. You can pretty much find it in all ENFJs to some degree, some more than others, depending on how focused they are on their P function, Ni. I'm not sure how to explain Ni itself in a relation to this, except that its method of processing information is extremely precise when compared to Ne. It's more deep and probing, as all introverted functions tend to be. Look at some of these major dictators that have held sway over a lot of people. To get an example of the type of Ni presence I'm talking about, look at Carl Jung in his younger days, an Ni-sub INFp. It's very staunch, almost ascetic-looking.
Most beta NFs (ENFjs, INFps), tend to have a very kingly, elegant, aristocratic approach to the world. This is why it's easy to see them as the types most likely for an open dictatorship. Also, they can be very volatile, which explains why these open dictatorships tend to rise very quickly and fall in about the same amount of time when confronted with an alternative.
If you look at types like the ESTj and other Delta-quadra types (INFj, ENFp, ISTp), you'll find the same aristocratic outlook, except different. Whereas the Beta quadra members (ISTj, ENFj, INFp, ESTp) have Se, the deltas focus on Si. Any dictatorship that they have any part in would likely be more low-key. In my experience, most of them would profess that they value democracy and equality. But if you press them, a little deeper, you'll find that they enjoy a more centralized form of government. So with them it's just not as easily seen. Whereas a an unhealthy delta ST might make an attempt at manipulation by using humor or saying that they care about you and that they're trying to help you, Betas have no such qualms with just saying, "Fuck you" right off the bat.