Yes, I expect a game developer to be consistant in his work. It worked last week and since it's conception more than a year ago, why would I expect it to fail now? When using old/simple Windows XP, I have not encountered any of the problems/bugs others have experienced. However, I didn't pay for this game, I guess I can't expect so much.
By that way of thinking, the game should also work for DOS users...
It could be any number of reasons why it isn't working for you now.
Perhaps an update to the Unity Engine that doesn't compile properly or uses DLLs that do not exist in XP.
Could be your DirectX installation is out of date and since XP is no longer supported you have to try and do it manually since Windows Update no longer does it for you...
It could even be something that only affects your computer!
As for the others who are having issues with saves...
Well the save system has had a major update so some bugs should be expected.
I had no problem loading my savegame from 18 and continuing.
If you are having problems saving a game it is most likely an issue with permissions.
You should not install these games on the C: drive especially if you are running Windows10.
Windows10 has a very strict permission system that restricts writing to the system drive outside of the operating system methods.
I run all these games off the system drive and never have any of these issues.