Yes, you can buy native BTC on ThorChain, that's fine.
They use decentralized validators with collateral to transfer data between blockchains. This model is better than a simple centralized bridge, but still worse than a true atomic swap.
Decentralized validators need to be paid, when I tested ThorChain, the final commission of the system was around 2%, that's quite a lot.
Also a system with decentralized validators has an additional attack vector, especially when swapping amounts larger than the validators' collateral.