Samsung made some drastic design changes to their Galaxy S line with the release of the S6 and S6 Edge, including the switch to a non-removable battery and the inclusion of glass on both the front and back of the device. This means should you want to access that battery and replace it at some point down the road, those panes could become a real pain. iFixit assigned the Galaxy S6 a repairability of 4/10 mostly due to this fact, noting that it is next to impossible to replace the screen glass without damaging the device’s display.
The score is a drop from the Galaxy S5’s 5/10 repairability, both down from previous Galaxy S models that tended to be favorable for at-home repairs. Other issues with the S6 include the use of extra-strong adhesives holding the phone together and keeping the battery firmly in place along with certain repairs (like replacing the USB port) requiring the removal of the front display (see above about breaking glass and/or display panel).
It would seem the advice would be to avoid prying apart and tinkering with the S6 unless you have a pretty solid feel for this sort of thing (or if you don’t mind a voided warranty and can afford to buy a replacement). The Galaxy S6 is a thing of beauty, for sure, but it is a beauty that is perhaps best admired from the outside alone.