“You can also make “in kind” contributions (for example, securities you hold in a non-registered account) to your TFSA, as long as the property is a qualified investment.
You will be considered to have disposed of the property at its FMV at the time of the contribution. If the FMV is more than the cost of the property, you will have to report the capital gain on your income tax and benefit return. However, if the cost of the property is more than its FMV, you cannot claim the resulting capital loss. The amount of the contribution to your TFSA will be equal to the FMV of the property.”
The downsides are having to report Capital Gain, and losing the ability to report Capital Loss.
You just weigh those costs (tax owing for capital gain, or loss of tax credit for capital loss) to see if they are sufficiently offset by the expected returns.