That's all kinds of sketchy, least of all because of the taxes, but also because you're playings games with public stock that they or anyone else could manipulate.
I've used TransferWise for over $8K and think the service is fantastic. I wouldn't touch this with a ten foot pole and my git is screaming "scam!"
Not sure why there was such a knee-jerk reaction, but it's actually fairly straight forward and not scammy at all. You're performing two trades, using a money market ETF that trades at the currency exchange rate, and paying standard brokerage fees + spread instead of currency conversion fees.
If you pay $20 in fees + spread for 2 trades, that'll be the cost of the conversion. For a $10k conversion, that's 0.2%. Unless you're suggesting that a money market ETF is prone to manipulation, your brokerage is going to scam you, or you don't like to deal with taxes on your stock purchases/sales, there isn't much drawback other than time. The larger the amount though, the more it'll potentially be worthwhile.
The only downside I can see is that it takes a fair amount of time (about 7-14 days) from the moment you move money in to the moment you get your exchanged money out. It'll take your brokerage several working days to "journal" your shares over to the other denomination, and the shares can go up/down in the mean time, generating capital gains/losses. Otherwise there's no catch.
I've used TransferWise for over $8K and think the service is fantastic. I wouldn't touch this with a ten foot pole and my git is screaming "scam!"