>I've had many more stability issues with the LTS versions of Ubuntu than with Mavericks or even Yosemite.
By any chance are you running OS X on Apple hardware? If we took OS X and installed it on an Ubuntu Certified Dell PC we may find that it has the stability issues, so if you are comparing it to OS X running on Apple hardware it's not a very fair comparison, as it's being run under ideal circumstances and Linux is not.
that may be "theoretically" the fair comparison. but in practice, most people run osx on apple hardware and linux on any hardware. So to get an actual comparison of the user experience, it is fair to compare them on different hardware.
I think it would be much fairer to compare OSX on Apple hardware, to Ubuntu on hardware where it's certified by Canonical. Meaning that the hardware works and you're using the version of Ubuntu it was certified with [1]. An example of this would be the Dell XPS developer laptops, which are certified, and an image is created that by Canonical with the correct drivers in it.
In those circumstances you shouldn't have any "stability" issues because it's a like for like set of engineering - basically.
In general, in terms of your other comparison with "any" hardware on the Ubuntu side - the two most likely causes of problems are hardware drivers and user fiddling (ie mixing repos and kernels). Lack of engineering support from manufacturers for client hardware is a big issue that's outside the Linux distros control. Nonetheless, if you use common supported hardware and standard installs then you shouldn't be having stability issues.
By any chance are you running OS X on Apple hardware? If we took OS X and installed it on an Ubuntu Certified Dell PC we may find that it has the stability issues, so if you are comparing it to OS X running on Apple hardware it's not a very fair comparison, as it's being run under ideal circumstances and Linux is not.