Critics and users alike love to throw shade at those built-in macOS and iOS apps—Mail is often panned for being out of date, most of us don’t want (or need) Stocks, and Maps still can’t hold a candle to Google Maps, four years after the latter was unceremoniously evicted from iOS 6.
For me, Calendar is the weakest of Apple’s built-in apps (with Reminders a close second). While the Mac version is passable enough, the iPhone app is borderline useless, even with split-screen month and list view enabled.