I attempted to build a systematic comparison of these three banks against actual small business banking needs. It was messier than I thought it would be.
The needs I tried to evaluate: having truly separate accounts for cash allocation, team cards without paying for a premium plan, being able to call someone for support, FDIC above $250k, no minimum balance, no monthly fee at the base tier, reasonable ACH speed, accounting software integration, real-time internal transfers, and some level of access control for a bookkeeper or accountant.
Here's where it got complicated: these products aren't trying to do the same thing. Mercury is optimized for startups in the VC ecosystem with strong developer tools and a beautiful interface. Bluevine is primarily an interest product with a checking account attached. Relay is focused on operational banking infrastructure.
Scoring them against the same criteria felt a bit like comparing a sports car, a savings account, and a pickup truck. They serve different purposes.
That said, if the criteria above are what matter to YOUR business, one of the three covers significantly more of them at the free tier than the other two. I'll let you figure out which one since my list of criteria is going to be different from yours.
My actual takeaway was less about the scores and more about the importance of defining what you need before you start comparing. The comparison only works after you've done that part.