So essentially we have two words that are designed to mask offensive things, you know, in a sort of overly sensitive and pussified way.
As for cuck, well, if you like your wife fucking other dudes, then you're probably fine with that. It's a whole genre of porn, you know. Just not sure what it means in the realm of politics. May as well call someone a kazoo technician.
Oooh. Bet your wife is banging other dudes, you ... piano enthusiast!
What They're Trying to Say:
This is essentially the liberal version of the snowflake insult, although paradoxically, you'll still see people on the right use it, as in "No, you're the Nazi!" because schoolyard rules regarding rubber meeting glue are how all great political name-calling matches are settled.
The Nazi is, of course, a Nazi. You know what a Nazi is. But there's debate aplenty over who qualifies as a Nazi and that's why it's an insult because no one, save a very few curious individuals, want to be identified as Nazis in 2017, no matter what they believe.
The fascist tag is very Nazi-esque but a more broad and judgmental insult. Nazi is what you are but fascist is who you are. At your core, you're a freedom-hating, intolerant bigot. And then, of course, you can just toss in "zealot" or "Trumpeter" or "Putin's Cock Socket" or whatever else besmirches the essence of conservatism inside someone.
Why it Fails:
Without question, if you dare invoke any of these insults, you'll be met with the one-two punch of predictable responses: 1) "Can't think of a real argument to come back with?" and 2) "No, you are!"
Nothing convinces someone you're full of shit faster than lobbing out a negative criticism of their political leanings. Because, as we've come to learn in this zany modern era, the one thing no one is ever wrong about is their own politics. God have mercy on those who believe other things because those fuckers are dick-brained Martians who eat paint chips. This is an easy position to hold because it applies to literally every spot on the political spectrum. Everyone is right about what they're right about, everyone else is wrong. Without fail. And if they come at you with a critique of your politics -- Nazi, fascist, communist, Libertarian, constitutional monarchist, well, fuck that. You just don't get it. You have no coherent thought so you're just tossing out buzz terms you heard on Twitter.