So, the network has generally become much more expensive to fail. It currently employs something like 10x the K computer (Top 1 in the Top 500) worth of compute. There are probably a few organizations that could get that compute power together for a while, but you'd have to ask 'why?' and 'for what reason?'

If you want to send a lot of money through Bitcoin transactions, I suggest you estimate expected compute cost of subverting the number of validating transactions you are going to wait to secure it, and using that as a baseline.

E.g. subverting 10 transactions in a row with > 50% likelihood (but doing so without anyone noticing, that is, timed appropriately) would take something like 10x the current compute power for about an hour, = close to a million dollars? hundreds of thousands certainly.

Ergo, wait a day for a $1mm bitcoin transaction to clear, and similarly, don't worry about a $500 transaction at all.