Why do new OS X upgrades have some issues for some people only?

I can only think of things like the age of your Mac, how much RAM ones has, third party apps conflicting, or third party hacks, but why do people have different issues when they upgrade to the newest OS X. For instance, I don't have issues with WiFi, yet so many others do. With my MBP non-retina, the font rendering if off majorly.

My AirDrop is fickle in that I can send from my iPad or iPhone to my MBP, but sometimes not the other way. It works and then it doesn't.

I think a lot of casual users probably don't even realize that their update to Yosemite made their Macs buggy and may even go to a tech store to get their Macs fixed.

Apple doesn't usually admit to bugs openly so I think most people don't have a clue unless frequently sites like MacRumors.

Any thoughts on why we have different performance issues when upgrading to the next OS X?

I do think OS X is stable overall. I have noticed that with each upgrade to a new OS X name, I had experienced some issues and this causes me some concerns for future updates. It could be hit or miss whether my specific issue is fixed and this ultimately affects my work. I have noticed that Apple tends to take their time addressing certain bug issues that aren't so called blaring.

I was new to Macs in 2013 since I was with Windows for decades. I was very happy when I switched to Mac as I do so much more and I found myself wanting to learn all I could. I'm now fully entrenched in the Apple ecosystem and I'm happy with it. As it stands, whenever I go on my Windows laptop for nostalgic reasons, it looks so crappy, I'm limited in what I can do, and it is slower than my MBP even though the specs are fairly comparable.

Anyway, with Mountain Lion, I had Gmail in Mac Mail sending out of different accounts and that was frustrating to troubleshoot, but for the longest time I didn't realize that it was an OS X and Google email exchange issue and all the while my some of my tech friends had no issues with Gmail accounts. With Mavericks, I had almost daily split second beachballs and I tried all the fixes, but it wasn't until a certain Mavericks point update that fixed it. I have zero beachballs in Yosemite. Now, some people didn't experience any of that.

I don't have many third party apps and I keep my MBP "clean". I don't do programming and am mostly a user. With Yosemite, I did a clean install as did many others, but they have constant WiFi issues and I don't. If I have the occasional WiFi issue, it's directly related to my Internet Provider and they have admitted that.

Also, other Mac users that have non-retina displays are quite happy with Yosemite and find the system font display crystal clear.

So that is what I don't get. Wouldn't certain upgrade issues be affected by all or is it that some people just put up with it or aren't bothered?