Owning your system is not free of problems, and anyone who tells you otherwise is selling.
You own the maintenance. When a payment processor changes its API, meaning the technical connection between two systems, someone has to update your code. A platform vendor handles that across all customers at once. With a custom build, it's your line item. Budget for it or you'll get surprised.
You don't get the ecosystem. Big platforms have hundreds of prebuilt connectors and a marketplace of add-ons. Custom systems connect to whatever you pay to connect them to. If you rely on a wide web of third-party tools that all happen to have native integrations with one platform, that's genuine value and you should weigh it.
Compliance features are another real gap. Platforms serving regulated industries carry certifications, audit logs, and reporting formats built to satisfy specific regulators. Rebuilding that from scratch is expensive and slow. If you're in healthcare, financial services, or anything with heavy inspection requirements, check what you'd be recreating before you commit.
And custom builds fail when the client can't decide. Software has to encode a decision. If your leadership team can't agree on how something should work, no developer can fix that. A platform at least forces a decision by removing the options. That's a legitimate benefit of buying, and we've seen companies who needed exactly that.
Last one. If your process is genuinely broken and you're paying to preserve it in custom code, you've bought an expensive monument to a bad habit. Change the process. Then decide what to build.