The important difference is the turn around time -- with Expo I can make a change locally and see it with my testers in less than a minute, whereas with TestFlight everything has to be manually reviewed for 2-3 days after a heavy-handed submission process.
I routinely do TestFlight builds without manual review and fast turnaround from build -> running on beta user devices.
PS: I'm using an iPad Pro 10.5 over LTE right now with an original Apple Wireless Keyboard 1 with proper arrow keys. I use Vim on it and can save to shared services. Heck, you can even fire-up the embedded python with :!python3<enter> It also redirects phone calls from my vintage unlimited data plan, so I don't need a phone... but it looks goofy talking into my iPad John Oliver-style. ;)