On Immutable X, in contrast to other scaling solutions to the Ethereum network, 51% of attacks are unfeasible because it is not a centralized side-chain. However, it still has the native security of the Ethereum blockchain.
As mentioned before, this protocol uses zk-rollups; users trade assets on the second layer blockchain; however, the validity of trade is saved on the layer one blockchain (Ethereum).
This platform uses STARK proofs, a kind of proof technology that allows users to share validated data or perform computations with a third party (they do not reveal the data to the third party, though). STARKs are post-quantum-secure and provide better user security in spite of the higher cost.
This protocol chooses to stay on Ethereum since its team believes that Ethereum has always put centralization over short-term scalability upgrades. However, it has suffered from problematic gas fee spikes and network congestion. It is still the leading blockchain for NFT projects.
The problem with scalability on blockchains is that it decreases decentralization and leads to compromising security. Immutable X and Starkware cooperate to increase privacy and scalability to solve this problem.
Gas fees are not non-existent, but the platform pays for them so its users do not have to pay any fee.