The battery life in my iPhone 6s Plus has been getting worse and worse since I fell in he lake with it. I helping my dad put out our dock and it broke so I fell in and my phone went in with it. It’s not like the battery didn’t work, it’s that it worked so poorly at this point (over a year later) that I needed to haul my 20,000 milliamp portable battery everywhere with my phone.
It went fine. I have a decent amount of experience opening iOS devices (except not this one specifically) so I knew kinda what to expect. The only problem I encountered was that the middle adhesive pull tab broke so I had some trouble prying the old battery out because of that but otherwise it went without a hitch.
The only advice I have is to make sure you have a screw organizer (I have one that came with an ifixit iPad Air kit), be careful with the screen (I didn’t break mine but I see how someone could easily do so), and be careful with the battery adhesive pull tabs because if they break you’ll have a heck of a time removing the old battery.