We've already seen how PoS can be gammed (NXT),
NXT uses very old first generation PoS, basically all problems have been resolved at this point.
Plus there are other solutions to PoS as i mentioned.
You really think Eth would move to PoS if it was a bad idea? (Yes i know it will be hybrid at first and moved full post at a later stage.)
we've already seen how the tangle thing can be gamed (IOTA)
Have we? Source? I don't know anything about this.