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.
I do not want to argue between atomic swap dex vs thorchain style dex, I like both and use both.
Yes indeed, thorchain has security concern, it is relatively new. It has newly launched streaming feature that cut down trading cost, I think thorchain trading cost is 0.1% to 0.3%, quite competitive to CEX. But for smaller trades, atomic swap based dex atomicDEX is better.
AtomicDEX or any atomic swap based DEX has liquidity issue. This is big road block. No liquidity, no big traders comining. atomic swap DEX is only for micro trades, or small retail traders unfortunately, this is not likely to change soon.