There seems to be a consensus of some people here that PoS is not secure, but I didn't see anyone elaborate as to why. Is it the monopoly problem or is there something else?

Please (really, pretty please) don't send me to Google it. I'm a newb, I admit, but I also spent a while reading the PoS vs. PoW posts and articles, and couldn't find any other consensus argument beside the monopoly problem.