Truth is an absolute legal defense to slander or libel charges.
As is honesty and belief that the person accused of defamation is disclosing information in the public interest, regardless of merit or soundness of their claims. I doubt any case concerning FE would revolve around the soundness of FE, but rather whether or not the accused believed she was acting in the public interest.
In the USofA, the most hilarious of legal systems, matters become even more difficult - a public person who believes she has been defamed has to prove that the defamatory statement was published with knowledge of its falsehood.
Naaaactually, it's not that simple as Alex Jones of Infowars is currently finding out.
Your opinion is protected. If you say, "It is my opinion that Astronaut Zeb is a buffoon with ears like a donkey." that is your opinion and it is protected.
But if you say, "It is my opinion that on April 12th, Astronaut Zeb lied about his participation in a 2016 space flight." that is an accusation.
Also, you can be held liable for reposting defamation even if you thought is was true.
In a brief for the Alex Jones case a first amendment scholar wrote,"It (posting opinion as facts) would allow unscrupulous news organizations to couch their language as opinion and mask their meaning with implication and insinuation. That would leave readers clear as to the message but avoiding all liability for defamatory remarks. This should not be allowed and is not allowed."
One day, an FE youtuber may go too far talking smack about Elon Musk and he will send a horde of zombie lawyers to lay waste to everything in their path.
In America, you can be sued for anything at anytime no matter how ridiculous the cause.