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.
Kucoin is not in the list, what do you think of them? I would suggest to spread your portfolio to more than one centralized exchanges if you are comfortable with them.