Native smart contracts would be cool, but Stacks is not native smart contracts. It's a separate blockchain that integrates with the Bitcoin blockchain.
In other words, you are trusting the Stacks blockchain to custody your bitcoins, just like with TBTC or RENBTC or any of the other decentralized Bitcoin bridges.
From their site:
> Like Bitcoin, Stacks is a layer-1 blockchain. Proof of Transfer connects it to Bitcoin with a 1:1 block ratio.
In other words, you are trusting the Stacks blockchain to custody your bitcoins, just like with TBTC or RENBTC or any of the other decentralized Bitcoin bridges.
From their site:
> Like Bitcoin, Stacks is a layer-1 blockchain. Proof of Transfer connects it to Bitcoin with a 1:1 block ratio.