Last year I bought a new Macbook Air that came with Yosemite, so after I got home and went through the setup process, I tried to update it to El Capitan through the app store. The laptop went to reboot... and didn't come back up. I thought it was completely bricked because pressing the power button did nothing: no LED light, no startup chime. Apple support guides said to hold down the power button for ten seconds and then re-press it, which at least made the laptop turn on and chime, but nothing happened beyond that. Booting into recovery mode didn't work, but luckily booting into internet recovery mode did. I had to reinstall Yosemite over the internet, create a USB image of El Capitan, then install from the USB.

Even after that, things never went smoothly. There was some sort of bug where I couldn't enter my Apple ID security answers in the OS dialog box. Like you, I had problems with external displays, especially with waking up a display when the laptop itself woke up. The very first time I tried to restore from a Time Machine backup I got errors. Problems with iTunes and Photos sync. Not to mention more or less subjective issues I had.

I had more problems with this laptop than I've had with any Windows machine.