Lightning Network
Payment channels with routed HTLCs. Strength: low-latency BTC transfers. Limit: liquidity management and routing for large or uncommon paths. Best understood as a payments rail, not a general compute layer.
Stacks
Brings expressive contracts anchored to Bitcoin’s security story with its own economic and consensus model. Evaluate developer activity, wallet UX, and how much real BTC economic exposure you get vs tokenised representations.
Rootstock (RSK)
EVM-compatible sidechain with merge-mining ties to Bitcoin miners. Useful if you need Solidity ecosystems. Understand bridge and federation assumptions — they matter more than marketing “Bitcoin DeFi.”
Ark / VTXO-style proposals
Focus on scalable off-chain transfers with different trust and uptime assumptions than classic Lightning. Promising — but watch operator models and how much sovereignty users retain in production deployments.