(IANAL) It's not about how it's stated, but whether it can be objectively proven to be true or false. "unsafe" refers to the likelihood of something bad happening in the future. You can't prove that something bad will happen in the future, so it's opinion.

Also not a lawyer, but that makes intuitive sense. If I say "that food tastes bad", it's phrased as a fact, but a "reasonable person" (which is in fact a legal test used for some things, although I admit I'm not sure about libel) knows that there's an implicit "...to me" qualifier because the concept of taste itself is inherently subjective. My instinct is that while there are some things everyone would agree on as unsafe, it pretty quickly turns into a judgment call, and it probably makes sense to allow even ill-informed opinions that are made in good faith rather than malice or negligence. The question then becomes whether there's sufficient evidence to conclude something like that, and while the bar is lower for a libel claim than something criminal, it's still not obvious this would be provable here.