The team quickly discovered that any sort of repair beyond the outer glass lens (for which GoPRo does sell replacement kits) is basically impossible thanks to the manufacturer's waterproofing efforts. That's a pain. The Session's battery, which is pretty small at 1000 mAh is likely going to need to be recharged more often than the Hero 3's power cell, thereby degrading faster and requiring replacement sooner. Except you can't replace it due to the waterproofed case.
As for optics, the iFixit team found that the Session sports the same f/2.8 lens as the standard Hero4, as well as what appears to be the same 1/3.2" sensor format that the old GoPro Hero employed. Overall, the Session proved to be nearly impossible to take apart and even harder to put back together again.