Tech is fickle by nature, and any variation in hardware or software can cause rather unexpected consequences. Take the recent launch of iOS 8 for instance, most people will consider it to be an improvement on iOS 7 — but others are already having issues that ruin their mobile experience.
Users have reported problems with the battery draining faster than it should, and with Wi-Fi slowing down rather considerably. But that’s really to be expected since every version of iOS has some teething problems when it’s first released. The problem is that a number of users are experiencing serious battery drain with minimal usage, even people who own the iPhone 6.
The forums for Apple and multiple Apple news sites are currently filled with complaints about serious problems with iOS 8, and despite some people finding fixes for things like the slow Wi-Fi, the majority of people are still suffering.
Hopefully the fact that this is the eight major iteration of iOS means Apple is already working on a fix for all these problems. Until then you’re better off waiting before you upgrade or having a charger handy.