yes they are very secure, but not on all exchange sites, large exchange sites are usually very secure, hackers are not easy to penetrate...these are some big exchange sites that are very hard on hackers attack = HitBtc, Kraken, Poloneix, Binance and Bittrex, this site is all I highly recommend.
But that's the whole point it's not easy for hackers to hack them but that's always a possibility. No exchange is safe.