Well most apps take care of that for you. I'm on a different instance and all I had to do was hit the reply button. For most people setting up the account and choosing server is the hardest part. If I handed someone my phone with an account already made they wouldn't even know they are viewing and commenting on different servers hosted by different people. They would just think its a different looking reddit clone. But I don't agree that logging in if your not on your home instance is hard as it requires you to find the post again. Some apps have an option to pull up the post on a different instance but last I checked that's on an option on the web browser.