Changes to MSSQL System databases from Cumulative Updates

What sort of changes are made to System tables (master / model / msdb) by Cumulative Updates? For context, I have SQL servers that have AlwaysOn Groups, and am looking at a way to roll back updates if it causes issues. All data and log files are on dedicated volumes, and SQL is installed on the OS drive. I have been testing a rollback process where I take a snapshot of the OS volume before installing the CU, and then update. After updating, I am able to restore the OS snapshot image back, and SQL shows the pre-update patch version. It looks like everything is working as expected after the rollback (i.e. the program files / DLLs / registry are all reverted), and not seeing any errors, but not sure if I am missing something or there is something that i should be aware of that might break something or be a risk. Thanks in advance. SQL Server version 2017 (im testing this is an old environment i had set up somewhere), and have tested this on a CU22 -> CU31 update + rollback.