I haven't used an email program on my computer for years.
I considered going cloud-only on my private gmail account a while ago, but then I accidentally locked myself out of my account with no way to recover. Since then I'm careful with having a local copy of important stuff. Not that everything in that account was important, but still reassuring that I can upload my old emails into a new account without losing years of information.
That unification is one of the best features. I frequently turn emails into calendar events and email from calendar events to all attendees.
I agree that's great on the desktop and a functionality I use daily, but on my phone I really dislike how it's basically impossible to go back between an email and a calendar entry because it's the same app. I always thought some in-app split screen would be amazing.