It would take intensive research of MacInTouch and other archives to even begin to quantify an answer. My personal recollection is that every OS X and macOS release since 10.3 has had some foibles in the initial release(s).Snow Leopard wasn't as buggy, at least by the final release, was it?
My Snow Leopard Server 10.6.8 VMs are rock solid for my uses.
It does seem that, at least since 10.14.4, major Mojave problems generally revolve around things like Gmail authentication changes and Adobe Acrobat DC's penchant for crashing just because it was launched or asked to open a file. I see about a 50% success rate in opening, reading, and modifying PDF files without crashing. But this began well before Mojave. Apple-generated faults seem to be primarily removal/hiding of functions rather than simple programming errors.
I recently upgraded a Mini to the latest hardware version and Mojave 10.14.4. Instantly successful were all peripherals, Vuescan, Indigo (home control), Scansnap, Dyn Updater, Office 365, Adobe CC Photo, Malwarebytes, Keyboard Maestro, BBEdit, and Screens 4/Connect, along with the usual Apple suspects. I confess that I did migrate just the user accounts and system configuration, but did fresh download and installs of the third-party applications, along with the usual toolbox of utility programs. With the exception of adding a Thunderbolt to FireWire dongle, the new Mini was literally a drop-in replacement for the old Mini - every cable, even including the AC cord. (I did put the new MAC address in DHCP reservations.)
So, I am well satisfied with both Snow Leopard Server 10.6.8 and Mojave 10.14.4 as reasonably reliable and fit for purpose. And also super-happy with the Mac Mini 8,1 with i7, 16GB, and 500GB SSD, and multiple external drives.