Payload can be fully customized, and pretty easily too. If there is a feature you're unsure about, describe what you're aiming for here and we can confirm if it's do-able with Payload
@jesschow probably can you give you some better direct information, but I think Payload is dynamic enough to handle most situations
FYI sounds like the Payload team is heads down this week on putting the final touches for the upcoming Launch Week, so you might not get an answer until at least next Monday.
Overall though I'd tend to say that unless what you need to achieve isreally
different from the Payload admin interface, you'd probably be wasting a whole lot of time reinventing the wheel by building a fully custom admin vs extending what's already there.
That said, like @thisisnotchris mentioned, don't hesitate to make new posts if you have a specific problem you're trying to solve. The more focused and clear it is and the easier it'll be for the community to help you! 👍