Mulit asset mode is safer. If you use a single asset mode, it that asset dumps, you will be liquidated.
If you use multi asset mode, if one of assets dumps but others don't dump, you won't be liquidated.
The reason I prefer multi asset mode this time is because cryptocurrencies are at their low price, when I tried it recently, I gained like passively as I also gain while trading. You can see how bitcoin and altcoin increased recently, I have them on the multi-asset account, but just little amount for trading. If it is during bear market, multi-asset mode will be more risky.
The use of multi-asset mode can vary from exchange to exchange, with some exchanges having stricter requirements for users to be able to access it. The $50000 requirement you mentioned is likely to be a measure taken by the exchange in order to reduce risk for themselves, as well as to ensure that users who access this feature have a certain level of experience and knowledge about trading.
But if I do not have experience about trading and I have $50000 to trade, they will unlock multi-asset mode for me and they do not care if I lose or not. Multi-asset mode is more risky because altcoins are part of the coins and the market can become very volatile at anytime. Probably they may do that for their own safety but not the safety of their users.
It's also worth noting that there are many crypto-exchanges available on the market, with different features and trading options, Binance is a popular one, but you can also check other reputable crypto-exchanges such as Bitmex, Bybit, KuCoin, etc. to see if they offer USD margin and multi-asset trading option, as well as checking their requirements and fees before sign up.
That is certain, I do check trading fee before I sign up on an exchange, it is very important. I also check bitcoin withdrawal fee. I have been using Kucoin for like 3 or 4 months now, it does not support multi-asset mode. I can not just be registering on individual exchanges to look for the exchanges that support multi-asset mode, that is the reason I created the last topic I referred to above, for people that are using the exchange that support it to let me know about the exchanges.