I don't think that Binance or Huobi is going to scam you for $250,000. They have been around for quite long. And the advantage with Bitcoin is that each and every transaction is recorded in the Blockchain and they won't be able to deny receiving the amount in their wallet address. But still I can understand your concern. A lot of the smaller exchanges are rumored to do such tricks to their customers. But reputation matters a lot here. If an exchange loses its reputation, then it will be very difficult for it to survive. And we all have seen what happened to the Livecoin exchange, when they had a similar exchange with one of the users (it was settled later).