Looks like batteries are the most difficult to manage for phone manufacturers but I don't understand why. Mobile chipsets are thousand time more advanced technology but they work much more stable..
SoC chips are far more predictable because they don't degrade as much as batteries do, they're more "solid"/"fixed". That's not to say they don't have issues, companies do have yield issues and defects but they're caught at the very beginning, not later. That's unlike batteries because their issues are often caught later after they're out of the factories, it's why Samsung was surprised. They wouldn't be stupid to sell bad batteries to destroy all of the momentum they had going.
Apple tends to have more technology in their batteries to ensure they don't overheat, charge consistently and so on but that doesn't actually resolve all of the chemistry issues.
When you have an unpredictable part, you're bound to have unpredictable issues with software.
In this case, it's not the battery that's giving Apple issues, it's the combination of software and battery. Apple is just at the point where everything is so complex that it is virtually impossible to fix them quickly and to get it right the first time. However, Apple can and needs to step up and offer free battery replacements, stop with these BS programs they have for iPhone 6/6S now.