So I'm not totally clear what you're asking. I think you're asking "what prevents a fascist takeover in other government styles / how does having a hierarchy prevent a fascist takeover?"

And genuinely to answer this it would be a massive essay. The easiest way to sum it up would be A) It doesn't, see the constant rate of falling into fascism or military juntas or other forms of extreme centralized authority and B) Decision making, which relies on a degree of speed and authority to declare.

If a crisis unfolds, teamwork is gonna be the thing that gets us out. That's basically the crux of the human story. When shit hits the fan working together usually gets us through. But to work together there's a degree of discussion and agreement that has to take place under a total autocracy where one person rules all they don't engage in much of a discussion and make decisions quickly, as they like. In a total anarchy government all people would roughly have the same opportunity to share their points or at least work together to make a decision. That's a rough generalization though, kings have advisors and anarchists would likely have some degree of representation because it's unrealistic to have one voice one vote on every single issue. But you probably can see where I'm headed.

Spanish anarchists ran into two huge issues repeatedly in this area. They'd discuss a little too much with no avail as once you reach a peak of discussion sometimes people just don't agree and there is no mechanism to basically force a decision. People might just ignore it. Representatives make a call for a decision or a push for a choice and if people don't agree, there's no movement in engaging with these decisions. And if any move was made to sort of curtail that issue, you'd fall into the "no true Scotsman" thing where camps and factions viewed themselves or others as moving away from the core belief of anarchism. Without that backbone of saying basically, "fuck you I'm in charge" you risk paralysis which appears all throughout any government you could imagine. Democracies today struggle with this when people point out how slow change takes, and often that can directly lead into a fascist slip where people either lean on power believing it'll get things done(Julius Caesar) or just a straight up overthrow of a government (Burkina Faso coup).

It might sound like I'm advocating for a monarchy or something but I'm not they face the other side with basically the same problem. If a king declares a decision and people don't agree now it costs energy, money and people to enforce it which is kind of what led to the dissolution of monarchies in the first place.

Governments don't last forever basically is the answer. They change sometimes for the worse. But having authority to make decisions and having people's input feel represented in those decisions are two major ways you prevent a complete collapse of authority. Anarchy as a functioning form of government just lacks an overall authority portion of that which really encourages one wing to step over another in favor of a decision being made and raising the degree of crisis that asks for a decision raises those odds.

https://theanarchistlibrary.org/library/jose-peirats-the-cnt-in-the-spanish-revolution-volume-1

This is a breakdown of the internal logistics of Spanish anarchists. It basically repeatedly brings up this issue without stating it or having an opinion. It will just state how representatives made calls for X or asked for Y to no avail, no solution was implemented. It sometimes mentions how certain decisions were entirely ignored. But does not offer the author's opinion on the matter. So it is not arguing for or against what I'm saying. It's just a good example of what those issues anarchy faces looks like in reality, if there is no authority to assert a decision sometimes a decision might not be made.