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Every comparison on this site has a company on the other side except this one. The real competitor is a developer who reads the list of capabilities, recognises most of it, and correctly thinks: I could write that. You could. The question is only whether writing it is the best week you could spend.
Build it yourself
When the backend is the product, or the product is small.
If what you are building is genuinely novel infrastructure, adopting somebody's opinions is a tax. And at the other end, if you need one table and one endpoint, a kit is a lot of concepts to learn for a Worker you could have written in an afternoon. Both ends of that range are real, and neither is a failure.
Use the kit
When the hard parts are hard for everyone.
The parts of a backend that take a quarter are not the parts that look hard. They are cross-rail entitlement resolution, a balance that cannot go negative under concurrency, a per-request locale, key rotation that does not break links already in inboxes, and an audit trail that can answer who did that. Each of those is a solved problem you get to skip.
Where building it yourself wins
No capability's opinion to work around, no upgrade to absorb, no seam that is not quite the shape you wanted. For genuinely unusual products that is worth a great deal.
Capabilities, bindings, migrations, environments and the config surface are real concepts. A single Worker with one route has none of them.
Any kit can be abandoned. Code you wrote is code you can maintain, and that calculus is not unreasonable.
A read-only API over one table does not need payments, matchmaking or an audit trail, and pretending otherwise is how simple projects get expensive.
If it works, the cost of moving is real and the benefit is speculative. Most rewrites are not worth it.
Write it yourself if the backend is your product, or if the product is one table wide. Otherwise the question worth asking is narrower than build-versus-buy: of the things on the capability list, how many will you eventually need, and how many of those would you write correctly the first time? Idempotent payouts, a locale negotiated from four sources, rotation that does not break old links, and an entitlement that survives a refund on a different store are each a week you will spend at some point. The kit's claim is that it is those weeks, and nothing more.