An instant crypto exchange is an exchange solution that aggregates prices and liquidity from multiple custodial trading exchanges.  Exchanges like Binance, kucoin and bittrex are called custodian exchanges.
Cryptocurrencies  and other funds like US dollar are safe or safer on instant exchanges because they are save on your wallets. You retain the full owner of your cryptocurrencies because they are save directly on your wallet in which you have your private key and seed recovery phrase. So, you have more control than on custodial/traditional exchanges. Exchanges can be hacked not like wallets that have private keys features.
But know that if you want to trade, you need to utilized custodian exchanges which means your funds are custodied at this time. 

Instant exchanges registration is very easy and you can sign up fast with just email address and a user name. It is more modern and easy to use.
But, the trading fees are higher than custodian exchanges. And not easy to use by novice that have little knowledge of how to protect crypto wallets.