The islands pattern is underrated for maintainability. I've found the biggest win isn't even the state isolation — it's that each island can have a completely independent upgrade path. You can rewrite one island from React to vanilla JS (or whatever comes next) without touching anything else.
The global state SPA pattern fails for a more fundamental reason than just being painful to maintain: it creates an implicit contract between every component in the app. Change one reducer and you're debugging side effects three layers away. Islands make the contract explicit — each one owns its data, full stop.
The one gotcha I've hit is cross-island communication. PostMessage works but gets messy. Custom events on a shared DOM ancestor end up being the cleanest pattern for the rare cases where islands genuinely need to coordinate.
The global state SPA pattern fails for a more fundamental reason than just being painful to maintain: it creates an implicit contract between every component in the app. Change one reducer and you're debugging side effects three layers away. Islands make the contract explicit — each one owns its data, full stop.
The one gotcha I've hit is cross-island communication. PostMessage works but gets messy. Custom events on a shared DOM ancestor end up being the cleanest pattern for the rare cases where islands genuinely need to coordinate.