Quality is not just about bugs
It's also about the user experience. I don't doubt that Apple are working to eliminate bugs as they come to light and have improved the robustness of their software over time, but the impression you get from the current version of iOS is just sheer sloppiness and poor design. It feels like Jony Ive has completely lost the plot. The interface has gone from simple to elephantine, with changes designed to put things like Apple Music front and centre being made at the expense of doing things as simple as enabling shuffle mode!
There are also bugs that have appeared and not been fixed. Not bug crashing failures that would show up in crash reports, but little things that nonetheless are irksome and make the quality of the software seem worse even if it is in reality more stable. For example, several versions of iOS back (was it iOS 8?) the player stopped respecting the "skip during shuffle" flag on music files. I use this to remove things that need to be played in a specific order or spoken word tracks (or silly little 1-2 second long tracks you get on some albums) from playback, so when a bit from half way through the third episode of Hitchhiker's Guide to the Galaxy, or Dante saying "I'm not even supposed to be here today!" pops up during shuffle (which is now really hard to find, by the way!) it breaks the flow of the music. This would be the easiest thing in the world to fix, but they've still not done it by iOS 10.0.2. In fact they seemed to have introduced another new bug which cause some albums that spanned multiple CDs to be treated as separate albums, requiring you to build a playlist to get them to play properly.
None of this leaves a good impression, regardless of how stable the software actually is.