Swap rates can make a huge difference and also can vary substantially between brokers. By the way VenetFx does not trade Bitcoin CFDs. What they do is accept accounts denominated in Bitcoin and also accept Bitcoin for deposits and withdrawals. AvaTrade and Plus500 do trade BTC/USD CFD's on accounts denominated in USD, EUR etc. The swap rates between the latter two are very different.

The counterparty to the CFDs is the broker who may typically hedge the trade. This does add certain risks.

By the way it is very easy to loose the entire investment with a Forex broker so it is paramount to be extra careful. To illustrate selling Bticoin short with 10x margin can wipe out an account real quick.