Why sending from an exchange account causes problems
Funding a swap directly from an exchange withdrawal is convenient and causes more than its share of trouble.
The amount changes
Many services deduct their withdrawal fee from the amount you asked to send. Less arrives than the order expected, which can land you below the minimum.
The timing is theirs
Withdrawals can be queued for review. A deposit that arrives after the window has closed has to be handled as a late deposit rather than as part of the order.
Refunds have nowhere good to go
If something needs returning, it goes back to the address it came from, which is a hot wallet rather than your account. Services are often unwilling to credit a deposit that did not follow their process.
What to do instead
Withdraw to your own wallet first, then send from there. One extra network fee buys you control over the amount, the timing and the refund path.