If anything, it looks shady - "outsmart your bank, use us, we do it super cheap, promise!" but no explanation of how it actually works...
TransferWise converts your money at the mid-market rate and matches you with people sending in the other direction. That's why it costs so little.
My understanding - after you've given the money you want to transfer to TransferWise, they find someone in your destination country who wishes to transfer money to the country where you are (kind of like a swap). This way, they cut off the banks and their fees and charge you a lower fee.
I imagine it's the former, but at the time I just used SWIFT - rather expensive, but guaranteed to work everywhere.