Wise use mid-market rates so no spread involved. They make their money from their fees which in general are percentage based. When sending currencies where the banks typically have a low spread, eg USD, there comes a point where Wise' percentage overtakes the costs of other methods. I think another poster worked it out at around 20,000 USD in his case but not sure of his Swift fees.
For currencies where banks have a bigger spread (bigger than Wise %age fee), the total cost will be less with Wise.