The two most famous documentary films of hers I saw were not terribly impressive even independent of the politics. But besides that, there is this weird martyrdom complex some people assign to her, like she was somehow the victim of what happened to her, and that is pretty loathsome. Far as I can tell she was a mediocre propagandist, that fashioned her not so passive complicity with Nazism into a crown of thorns. I think if she made the exact same movies in Britain she would be a footnote in a film history book.
I think what you're overlooking here is several key factors. I find her filmmaking style utterly riveting. The shots she uses, the way she films people, and the way it all looks is sort of the first of its kind. I think she definitely influenced modern moviemaking. Say what you want about the films (I actually think Triumph of the Will and Olympia are quite visually impressive) but what makes them such effective propaganda is subtly.
The politics of a film like Triumph of the Will isn't really given just from Hitler's mouth. However, he descends from the Heavens in an airplane at the start. The way she films the German soldiers, the up shots of Hitler, it's crafted in a way to show he is authority, he is given to us by God, and the German people have a massive, 'undefeatable,' proud army. It's a very good propaganda film because it isn't just juxtaposing Jews with rats. Frankly, that'd be tacky.
A good propaganda film that blends the in your face politics and great filmmaking is Battleship Potemkin though. Hard to beat Sergei Eisenstein. When the Cossacks descend the staircase too? Wow.
I just ousted myself as a lover of propaganda films. Well, the Soviets and Nazis did do them better than Frank Capra's "Why We Fight" series. Really like Capra, but bravo Eisenstein and Riefenstahl.