Considering which version to FPSC to use, then one should consider that Characeter AI and behaviours as well as issues with platforms, paths and ducts and other issues too are largely problematical throughout. Long standing issues or new ones they derive their inheritance from the past, those unresolved and by and large, though perhaps not exclusively, always having existed.
There is no reason to suspect that this will change in forthcoming developments of either currently existing versions or those of newer versions of FPSC apparently in various stages of development.
What one may consider might seem to be a relatively simple request for a fix to an issue or a feature, take physics and Ladders for instance may well never be realised as becoming a reality in any version as they may not be seen as of any priority to the developer though may well be to the users. There are often conflicting opinions between what TGC and users might think.
Users want to have the best tools to make the best games they possibly can. The developers may long for that too but their desires are tempered by overiding commercial constraints. Given that its no contest.
Unlikely that some major issues or features may ever get updated or fixed.
You may have to make do best you can with what you have or are given in the near future to say the least. With FPSC and many other TGC products being constantly under development and TGC having finite resources, instabilities, issues, bugs and the consequent requests for fixes to features, issues or updates are likely to abound in plenty ongoing well into the future.
At the moment Characters in X9 you should be able to get going up and down stairs but as these versions develop who knows what will improve or not.
As always what will happen when future versions are released one wont know until thats a reality, which could be a considerable amount of time. In the mean time you are stuck with utilising whats available until you become stuck with what you have it replaced with.
Not much choice other than to make the most of it or not.