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Im sure that cold wallets or hardware wallets are still safer than exchanges, their might be some issues in offline wallets that a lot of people encounter, and also there are some issues too when it comes to exchanges.
Still their are a lot of exchanges that you could also trust to invest your funds, also the only advantage in exchanges is you could convert/buy/invest your funds in other cryptocurrency that could potentially increase its market price, but when it comes to security hardware wallets/offline wallets are surely better than using exchanger as a wallet and it is not advisable when it comes to long term investment.
Probably because not all exchanges could sustain the market a big dump in the market could potentially bankrupt the exchange's website.