FYI Data point
I tried to do a 500k to Emirates from Capital one in January - my first transfer - then had second thoughts - I tried to cancel the transfer but was told it could not be reversed
Due to fraud the transfer failed - good for me! The miles came back and were used slowly
I found that 100k transfers went through to TK and EK with no issues and I did some to Aeroplan and some to SQ - the last one was 500k - went without issue but was after I had done a few transfers before
ALL my transfers - TK, EK, SQ, AC went through in seconds