My experience has been that every new release of OS X is more unstable than the previous.
I'd very much like to go back to Mavericks. It was the last release I used that seemed to be decently stable. Yosemite and El Capitan have so many problems.
* I've had to switch to a wired ethernet -> TB adapter and turned Wi-Fi off altogether because of Wi-Fi instability, despite having enterprise-grade WiFi APs in my house (Ruckus and Ubiquiti).
* Peaking out the CPU usage and GPU usage at the same time almost guarantees a kernel panic despite Apple's hardware diagnostics reporting no problem and my Fluke temperature probe not showing temperatures going above 90C (although to be fair, that's very hot... that's standard Macbook Pro temps).
* Crazy audio issues for SPDIF, despite it being a basic standardized protocol that's had kernel support since OSX was released, every time they come out with a new version they manage to make SPDIF output break in strange ways.
* Sleep/Resume/Hibernate no longer works reliably, many times instead of waking up the system will simply reboot.
* Bluetooth devices (specifically Magic Trackpad 2) will disconnect and then reconnect randomly for no apparent reason.
I don't even use any of the Apple provided software other than OS X itself, but I still have seen the product go rapidly downhill. I went from being an Apple hater to being a hardcore Mac user, to now back to being an Apple hater that's just waiting until I can find a decently reliable Linux ultrabook (Lenovo X1 Carbon maybe?). The repairability of a laptop isn't even a concern to me at this point, I consider buying a laptop a sunk cost of being in a technical job. I just need it to work reliably because I depend on it for my livelihood and it's currently unacceptable.
The other day I was working remotely and in a conference call with the CTO and had a kernel panic twice in a row because I was doing `vagrant up` in the background which kicked me out of the call. How does that look for my professional reputation when I have technical difficulties during a call and I'm supposed to be a senior level technical staffer? Totally unacceptable.