When people use the horseshoe theory, it feels as if they're only looking at politics like this:
Left < Center > Right
That's it. A very simple view of politics through the lens of idiots using it everywhere.
Reality is that there's a bit more to politics than just being left or right. A good example is the ideologues of authoritarianism and libertarianism. These are not limited to left or right politics at all and create vastly different governments, even if both were on the left or right.
An example being that if you have a leftist authoritarian government, you get a communist dictatorship ala the USSR.
A leftist libertarian "government" is basically anarchy, hopes, dreams and rainbows.
Something simple like that takes the horseshoe and flings it out a window. Those on the far left and far right will have vastly different wants for a government because they'll have different ideas on what the government should do in regards of the people- some may want a dictatorship and go full goose-step while others may want to bring back monarchies. Others still would love anarcho-capitalism and sign NAPs for everything. It varies greatly.
Where it does get it right is their methods: violence, terrorism, open revolt, protests, civil discourse, etc. The reason is because extremism is extremism- it's inherently violent because it's extreme.