There’s two basic models, custodial and non-custodial. The vast majority of options are custodial, and that means a third party has your coins. This entails counterparty risk.
What is a reasonable return to lend out your Bitcoin? Is 6% good? Is 15%? It entirely depends on the counterparty risk. You’re not earning money for lending out BTC, you’re earning money for lending to a specific counterparty like BlockFi.
So, what’s a reasonable yield? Junk bonds in the US have typically paid 4-6% over treasury rates. For a risky start-up, uncollateralized loans would typically be more like 15-25% over treasury yields.
Short-term treasury yields are currently 2.5%, so I’d look for ~22%+ yield to lend my BTC to a start-up at a minimum. Now, this superficial analysis is unfair to Blockfi and others who would rightly argue this isn’t a purely uncollateralized loan.
On their website, Blockfi says they “typically lends crypto on overcollateralized terms.” The specifics matter a lot here. If that collateral was legally owed to you as a specific lender, these should probably be viewed as collateralized loans. They’re probably not.
If you as the lender have no legal right to specific collateral, then you’re simply a creditor to a single company, Blockfi (or whomever else), and you might not even be a senior creditor (for Blockfi, I believe you are a senior creditor, but this should be confirmed for each platform.)
So the right way to think about making a loan on a platform like this is very similar to if a crypto start-up asked you for a loan and offered you some interest to borrow your money, and promised to keep a pile of cash representing their general obligations to investors.
Non-custodial solutions are very different. With these, you may potentially have zero counterparty risk, rather you’d be subject to some sort of liquidation risk and protocol risk. If the market gaps lower, the proceeds from liquidating collateral might not pay you back.
Additionally, using things like multisig solutions for a lending platform have serious risk of bugs. Just how serious? Ask Parity, undoubted experts on ethereum smart contracts, who twice lost tremendous sums to bugs in the simplest type of multisig smart contract.
TDLR: The growth in crypto lending markets is awesome for the ecosystem (separate topic), but is extremely risky today however it’s done, and I think the yields are far too low to price that risk from my perspective.