The problem with the beta version A/P was that it needs specific tuning for each aircraft (much as FS is partially so tuned via AIR and CFG file parameters), and investigating what parameters were needed and how to document them sufficiently for users to do the job for their favourite FS aircraft, was a much bigger job than Enrico had time for with everything else on his plate.
I tried to help by supplying faster feedback loops operating to supplied parameters but inside FSUIPC (hence inside FS), but the speed control was still a problem -- generally the Pitch/Bank/Heading guidance was good, not needing much tuning, but it was difficult to deal with the throttle to set correct speed without unwanted oscillations, at least without much more and specific tuning.
The feedback control facilities are still available within FSUIPC, though generally not published, and should work with FSX too (though totally untested at present).
I know that for PMDG, who do this sort of thing for very specific aircraft, it takes many many man-hours to get their autopilots just so. In fact this is the main reason they give for not publisihng an interface to it which could be better used in PM (for example).
So, you see, PM has a difficult problem unless you are going to restrict its use to specific models.
The way the default AP works in PM's MCP module does make use of FS's inbuilt AP modes, but it doesn't just set things and leave it to it -- it is constantly adjusting the parameters so that the A/P does what the MCP wants. It has always worked pretty well -- but you should be aware that the FSX default models are not the same as the FS2004 ones. The 737-800 isn't bad, but I still prefer the PMDG 737-700 and -800 air and CFG files, with some changes made by Thomas.
And the pilot must always take care to keep the aircraft operating within the A/P's capability envelope. Once outside chaos ensues!
Regards
Pete