Samsung must be pushing the electronics and batteries to the limit in the new models. I have a Galaxy Note 3 and it's an excellent smartphone, and has never shown any tendency to cause problems via overheating or via battery problems.
It's nearly 3 yrs old, been accidentally dropped from around 2 metres height multiple times, has a smashed screen (despite a rubber-lined steel protective case), and it still functions flawlessly.
What gets me, is the Note 7 doesn't appear to have any huge advances in technology over the Note 3, just incremental "improvements". However, the "improvements" are obviously pushing the battery limits.