The basic 'operation' of HVAC is rather simple ,the real problem is 'computers' are so dang cheap, that 'engineers' cut all kinds of 'code' for 'fancy' functions and looks that ,in the end do NOTHING for the safe,reliable operation of the equipment. Then after the design, the 'bean counters' take their 'kick at the cat' to reduce the cost by trimming 'this and that' with the end result a low grade product with a very limited lifespan.
My AC is a Clare unit, 36 years old, 2 ton, still works GREAT. ONLY reason I got rid of the 96.5% furnace(36 years old...) for a 94% one, was to get a deal on insurance.
As for the 'computer' control board. Yes ,propriatory but nothing that a $2 PIC,20$ worth of SSRs, and 100-120 lines of 'code' can't replace. BTDT back in '87, even had ultra high secure, hacker proof remote control via phone lines.
Today, a neighbour's RedLion brand shallow well pump died, it's maybe 3-4 years old ( you can eat of it it's that CLEAN !). $400 pump needs a 50 cent spring for the centrifugal switch. Red Lion can't supply the part, can't fix it, says gotta by a new one !