If using an external screen was built into both the camera and the screen/phone, this would be great. For example, using AirPlay between devices and the Apple TV works like a charm. It's one of those things that just works, and makes everything less stressful and way easier and each device more capable.
I have the option to use my phone as a screen for my GoPro and my Canon 80D. It's nothing but a pain in the arse. The cameras connect via wifi, and become their own wifi bases. Convincing my phone to connect to the camera instead of another network is a nightmare. They disconnect all the time at any excuse. It only works without any hitches about 30% of the time, maybe 50% of the time after multiple attempts, and maybe 20% of the time I just give up.
Then the apps are shoddy. Canon and GoPro use different systems and designs and UI ideas. It's a complete shit show.
I really like being able to transfer images and videos from these cameras to my phone to edit or share on Instagram and stuff, but I'll avoid using the apps for image previewing at every opportunity.
If Apple had a camera and phone connection that just worked like AirPlay, I might be tempted, but as a real solution to someone who uses a camera a lot, the current "phone as external camera screen" options aren't close to being ready yet.