I have been an orbiter fan since its early conceptions and way back then you could guarantee that if you downloaded an addon then it would work !
Nowadays I find that more addons fail than work - especially with the advent of new technology
That's because Orbiter has undergone 20 years of development. I'd say old DLL vessels are a hit or miss in terms of compatibility. Some CTD outright, others experience ground handling issues that actually can be mitigated. MFD's on the other hand have excellent compatibility. 20+ year old MFD's are still working. Celestial bodies addons from 20+ years ago still work. Surface bases can be
partially made to work: Just update the base cfg format from pre-2006 format to 2006+, remove surface tiles, add dx9 flt file, place the file in the appropriate Base folder in the parent body. The spacecraft/multistage vessels also exhibit decent/good compatibility for being 20+ years old. They are a god send because they allow vessels to be defined in simple open text format, and so with an updated dll they can be fully supported today. As it is,
spacecraft2+ vessels can be handled using the
spacecraft4.dll and compatibility is good for the most part.
multistage.dll vessels will exhibit issues with the stages/fairings jettisoning in the wrong position, but other than that work when using Legacy Touchdown Points.
All in all, although I think at face value the compatibility picture looks bad, a lot of issues are simple things like making sure not to override certain DLL's, cfg files, updating base cfg format, making sure
Earth.cfg isn't overwritten, etc. Things that good tooling will solve.