Glitches of Class Expansion in FEBuilder

I’ve been toying around with febuilder and learned that you can expand classes. But apparently it causes glitches? Can someone give me a list or how much they know?

If you extend the class, you will be unhappy.
Do not extend the class.

In order to extend a class and make it work, it is necessary to extend save data.
And it causes various mysterious phenomena.

Sudden game-over occurs when replacing a complex unit.
If you use a save slot other than slot 1, the game will be broken.
It may occur in combination with other patches.

If you can solve these phenomena yourself, there is no problem.
If not, do not extend the class.

There are many reusable classes.
I think you don’t need to do anything dangerous.