I have seen very very few egregious errors from Grok and most of the factually incorrect posts seem to be caught by it. For the ones that I Grok was incorrect - I verified it myself and I was wrong and Grok was right.
It turns out that Grok is actually really reliable. But the limitations are that it can't fact check extremely niche topics or intricate posts. But 90% of the cases it does catch them.
Edit: it also makes cute errors like this sometimes https://www.timesnownews.com/world/us/us-buzz/grok-ai-mistak...