They have introduced perpetual swaps recently, product is a bit raw, because there aren't stop losses, but I had a talk with support and they promised to deliver this feature asap. So I think after adding stop losses they can compete with bitmex. Personally I don't care about it, because I use futures due to absence of funding rate and I don't want to pay extra fee daily for borrowing assets.
Not having a stop loss feature sounds a bit odd because every platform offering a similar product that I know of allows you to set a stop loss. It's an important enough basic feature that a lot people happily use.
Then there are the fake spot volumes OKEx has been generating for quite a while now. For such a large exchange it's quite embarrassing to see a report expose them as the 4th most fake volume generating exchange.
-smaller fees (on Bitmex fees are incredible, especially on withdrawal and if you have small balance , withdrawal fee is getting major part of your deposit)
BitMEX itself doesn't charge a fee for withdrawals. The only fee you have to pay is a dynamic network fee, which means that during low network congestion the fee is low and during high network congestion the fee is higher.