Paolo, previously, for as long as possible I too held onto versions of MacOS and Logic that worked just fine. But more recently I've needed to know if the latest Logic and macOS would help, hinder or not affect my Situater subsystem, given that I was getting it close to release. But now I'm thinking it could turn out that I just keep Situater for myself. (In which case I'd lobby the Dorico team to do it for everyone else.)
I think it was Logic 10.3 or thereabouts that the Transformer's 14-bit arithmetic was broken; it could then do only modulo 256 arithmetic (i.e. 8-bit). I sent a new bug report about it to Apple for every new release of Logic in which the Transformer was still not fixed. Meanwhile I'd redesigned the relevant parts of Situater to use the Transformer in its broken state, although this meant Situater could no longer use 53 pitch classes per octave. But lo and behold! Ironically, in Logic 10.7 the Transformer can once again do 14-bit arithmetic!
I'd say version 9 was the the most recent good Logic. And Snow Leopard the most recent good Mac OS.
It seems Apple's constantly replenished army of young noob proggies and coders just can't help rushing to demonstrate the truth of the "Peter Principle" (i.e. that people rise to the level of their incompetence) - the result being what in Finland they call "uusavuton" ("the new helpless"). Also, perhaps Apple management are busy proving the truth of one of the corollaries of C. Northcote Parkinson's Law, under the chapter "Plans and Plants", in which Parkinson points out many historical examples of the perfectly purpose-built new building being a portent of the organisation's terminal decline and ultimate demise! Not knowing anyone working at Apple and not knowing a thing about how Apple employees actually work, these are of course merely the images I conjure to try to make sense of the often baffling outcomes of Logic and MacOS updates, Lol.