The last person in usually gets the best deal, in that they can get preference and push everyone else (previous investors, founders, and employees) down. If things goes south, they get their money out before anyone else.
But the pref stack always favors later investors, partly because that's just the way it's always been, and if you try to change that now no one will take your money, and later investors will not want to invest in a company unless they get the senior liquidity pref.
Its less financially/legally saavy parties like angel investors and early employees who (sometimes) get screwed out of valuation