PartyKit is extremely unopinionated. It's essentially lightweight javascript server, that launches fast and autoscales (I don't say this as as a bad thing, it's a useful primitive). Most people seem to run yjs in PartyKit, but you can also run automerge or even Replicache – my company's other project.

Reflect is entirely focused on providing the best possible multiplayer experience. We make a lot of choices up and down the stack to tightly integrate everything so that multiplayer just works and you can focus on building your app.

That's the plan anyway :).