The only downside I can think of is that the compatibility mode only delays the breaking change, and one day this change will also reach the compatibility mode (at least that is how I understand it – you can read the official post about the compatibility mode here).
I think the intended use of the compatibility mode is:
- Notice that some of your installed userscripts don’t work anymore.
- Report the problem in the respective threads.
- Activate the compatibility mode so that you can still use the scripts while the script authors hopefully try to fix them.
- Every once in a while, check if the scripts were fixed, and if they were, disable the script compatibility mode so that you are using the up-to-date version of WK.