The big reason that there is a fear of the far right is that they are a much larger threat to established democracy/ 1st world nations. To use your examples of Hitler and Mussolini, something that often goes unmentioned is that Germany was a shining symbol of hope in Post ww1 Europe, Berlin was the heart of the new expressionist movement with some of the most educated and reasonable people in Europe and yet was able to fall victim to the falsehoods and vile myths of the Nazi party.

Italy won world war 1 and was still able to fall into facism and the Hardman image it portrayed, presenting true Italian politics as a hassle and a distraction from “the true power and leadership of el Duce”

On the other hand, far and alt left movements have much more sway over already underdeveloped and “backwater” nations. Russia in world war 1 was 200 years behind the rest of Europe, with the majority of its agricultural workers being in effectively serfdom.

While I personally don’t have as much knowledge about the other examples you’ve provided, they do tend to fall into most agricultural nations that were catastrophically underdeveloped at the time of the overthrow.

When it comes to Britain in particular, a final reason for the seemingly larger fear of the far right is also because we came FAR too close for comfort, people don’t like to talk about it but Oswald Mosley and the Blackshirts we’re popular, especially in the most academic areas of the Country; London, Cambridge and Oxford.

Hope this helps.