> However, they criticized it for having stability issues and overall sluggishness
> iPhone 3G users reported performance and battery issues after upgrading to iOS 4
> The iOS 5 update was the subject of criticism for iPhone 4S users, as the upgrade caused problems with battery life, failures of SIM cards, and echoes during phone calls.
> A study by Apteligent (formerly Crittercism) found that the rate at which apps crashed in their tests was 3.56% on iOS 8, higher than the 2% found on iOS 7.1.
> Forbes published several articles focusing on problems in iOS 8 regarding Wi-Fi and battery, Bluetooth, and calendar.
A lot of iOS9 issues received media attention:
Except for iOS6 & 7 which appear to have had a milder reaction, if anything I think iOS 11 may be a more stable release compared to iOS 8, 9, & 10 in that while there are more frustrating UI bugs you can hit (auto-correct, iMessage message ordering) but the overall performance & stability hasn't suffered as badly. Also iOS has knock-on impressions from OSX stability which has had similar UI embarrassments + a couple of security-related missteps.
My point is I don't think there's any single particular cause that's identifiable as the reason behind poor SW quality. If it were the hundreds if not thousands of really smart people Apple employs would have solved the problem. At this point it's all higher-level issues & more often than not tradeoffs (e.g. reducing the features you ship every year & going away from a waterfall development model)