Well, that is a poor excuse since a lot of their software problems end up being ridiculously obvious issues hours after a public release and not strange fringe cases found weeks or months later. Also Apple has a beta program set up months in advance that gets into the "public" users hands and yet does not identify many glaring release issues.
Apple also controls their own hardware and has a limited range of product variations to target their software on so when Android or Windows can release on a much wider set of hardware and has greater initial quality and performance then many recent Apple OS releases, it just speaks volumes to the actual level of quality of Apple's development processes.
Lastly a trillion dollar valued company should not have a "small" development team incapable of delivering better initial release quality, not for something a prominent as the software required for the hardware that made them a trillion dollar company. I don't think this comes down to incompetent developers, but I think obviously there is a culture of poor executive leadership and overall denial at Apple where they think they are still producing the kind of quality they were known for back when Steve Jobs used to chew the heads off his development team when the color of an icon didn't come out right.
Excusing Apple for the plethora of iOS and Mac OS release bugs is nonsense. The company wants more money for their products and so my expectations of initial quality is, and should be, far higher then the average software company. If Apple wants to sell $300 phones full of bugs then my expectations will match the value of the phone. But sell me a $1300 phone and it is quickly broken or crippled by the next iOS patch or major release is inexcusable.
I would agree ONCE it is released to the public. This is BETA software - that means that it's not done yet. The reason you have a BETA is to fix bugs -- If your not reporting bugs and you're in the BETA your part of the problem. Software development is a process and a few BETA cycles are part of that process..... Overall iOS 12 is Apple getting back to quality and bug fixing to improve performance, and not add as many new features. I agree that QC and overall quality been slipping the last few years (It appears that Apple does too with this version), but this BETA has been the most stable to date, as its mainly about getting bugs fixed.