The errors I ran into in 39.3 are very few and I have been able to fix those easily. I even posted them here. The mod I am using however isn't the same as what most people are using. I tend to think a good bit of the errors are in relation to the mod people are using. Give a number of them have "game/Mods/" in the file paths of where they are having trouble that tends to support that view to an extent.I just want to bring to your attention, Vren is working on some testing framework for 0.40, maybe it's just best to write off 0.39 as a loss and wait for the next one?
That said there are a number of tools I discussed on here renpy developers could use to reduce errors. It isn't just that learning some better coding practice can help also in that area.