Sierra used to have a russian language support board(not anymore). Someone said in another thread they are from Ukraine. I have psychological problems surrendering passwords to a closed source software from unclear origins. I have used them for charts only, that part is quite good in my opinion.
There was a futures broker bankruptcy like 10 years ago (the thread about this is still burried somewhere in this forum), and the court documents showed that ~90% of clients lost money. That is by the way way worse than Oanda, who must disclose the number of profitable/lossmaking client...
https://www.interactivebrokers.co.uk/en/index.php?f=1595
For a FX position you will be payed/charged the interest rate differentials on both legs. Look at interest rate payed/charged in the link above. You get nothing for your 21.5K USD leg and should pay like 3% for the ~500K CZK leg.
At least at cboe futures premium over spot was all over the place anywhere from 4% to 8% . Didnt look much like efficient price discovery. Then again spot btc doesnt either.