Not necessarily true. iOS 6 was riddled with bugs. iOS 5 actually had so many bugs they released an update specifically to address battery life, which they haven't done in a while. I can't say anything about below that but I do remember iOS missing a lot of core functionality for quite some time.
Battery life issues in iOS 5 I would've been unlikely to notice because I only had an iPad at the time and they last forever anyway.
The thing about buggy software, is that even if one version statistically has more bugs than another version, if the version with less overall bugs has more bugs that affect the user personally then it will appear more buggy to that user. I hope that makes sense.
iOS 8 has a lot of bugs that affect me; far more than any other version I've ever used. Hence my dissatisfaction with it.