The Polkadot network itself is extremely secure thanks to its employment of ‘shared security.’ This allows any chain that becomes a Polkadot parachain to benefit from the security provided by the main Polkadot Relay Chain validators.
This is a big advantage over many other interchain protocols that build on bridges and require each chain to maintain its own validator and security. This is because, the bigger the validator set is, the harder it is for bad actors to take control of enough of the network to mount a so-called 51% attack.