Is keeping crypto on exchanges an absolute no-no despite proof of reserves?

The exchange I am using has ensured proof of reserves: "'exchange name' holds all customer assets on a 1:1 basis."

I want to gain interest on my crypto while holding which would be a significant amount when I cash out, however, not sure whether I should do this despite the reassurance from the exchange - I am using a cold wallet. Is exchanges wallets an absolute no?