I've been reading an article about arbitrage in Bitmex with Buy Spot vs. Sell Futures. The mechanics are very simple:
Find a basis between spot and future.
Buy X amount of btc on exchange.
Sell the amount of X + the basis.
At settlement date you win the basis whatever happens. (plus your initial balance).

If been doing some numbers and I don't see the arbitrage. If the price goes up you lose everything (you got margin called).
Am I missing something. Thanks!