RainbowKit 0.12 uses Wagmi 0.12, which uses ethers.js under the hood.
RainbowKit 1.0 has been released, which uses Wagmi 1.0. Wagmi 1.0 switches from ethers.js to Viem, which results in some significant changes. Additionally, RainbowKit 0.12 supports WalletConnect 2.0, which is the main goal of this work. As a result, I'm deferring the RBK / Wagmi 1.0 upgrades for now.