Third party apps don’t have the ability to back up in the background all the time the way the native Photos / iCloud experience does. They need to be periodically opened to have temporary background access.

Launching the third party camera app cannot be done from the lockscreen.

What you’re not understanding is the entire point of folks’ complaints. With arbitrary restrictions put in place by Apple, there cannot be full parity in functionality between Apple’s native apps / cloud experience and those that can delivered by third party apps. While it’s possible to use third party apps, there are a bunch of little quirks and inconveniences that will ultimately drive the user back towards the native apps and spending money on Apple’s cloud service.