RSK is pretty much like Ethereum blockchain as far as use-case is concerned. You can create complex transactions, write smart contracts, lend or borrow like DeFi dapps, etc. In-fact, smart contracts are written in Solidity only just like Ethereum chain.

The major difference which makes RSK different from Ethereum or other blockchains is the way network security is maintained and how it is linked to Bitcoin. RSK secures its blockchain with merge mining which means Bitcoin miners can include the merkle root of RSK transactions while mining and use the same computational power for RSK as well. Since, the difficulty level is lower for RSK network, Bitcoin miners can quickly find block solution for RSK so new RSK blocks can be created quickly than Bitcoin blocks which means more scalability, more transactions and less fees.

In order to move bitcoins to RSK network, an independently executable smart contract known as Bridge Contract is used. A user sends bitcoin to multi-sig address. The private keys of the multi-sig address (better known as Bitcoin PowPeg Vault on RSK) are stored in special-purpose microcontrollers (inside PowHSM device) which virtually makes it impossible for any party to know them. On top of that, PowHSM device is included with SPV node. So when user sends fund to mutli-sig address, bridge contract builds the RSK transaction with equivalent sBTC, then sends the unsigned transaction to PowHSM devices. Next, the SPV node of the PowHSM independently verifies whether enough confirmation has taken place, if yes then independently signs the transaction using private key, sends the signatures to Bridge Contract which accumulates all signatures from different PowHSMs and create the final signed transaction, broadcast it and user gets the equivalent sBTC on his RSK address. Withdrawing the funds (known as peg-out) works the same way in opposite manner.

Since, 90% of the Bitcoin miners today are doing merge mining, out of which 60%+ are doing for RSK so it is safe to assume that RSK sidechain is securer than altcoin blockchains. Morever, in-built SPV node and tamper-proof micro-controllers make functionaries of RSK superior to other sidechains. So, on whole we can say RSK is one of the best alternative available today for smart contract creation.

RSK is still in development phase. The process of peg-in and peg-out which I explained couple of paragraph above is known as PowerPeg protocol which is only-recently introduced. Earlier it was using centralized custody of funds by multi-parties on Bitcoin side. That maybe the reason, RSK doesn't have wide acceptability like Ethereum yet.