As described in Clean ABAP:
- No instantiation. You cannot create multiple instances of the same function group.
- No inheritance. You cannot inherit from or let inherit function groups.
- No interfaces. You cannot provide two implementations for the same function group.
- No substitution. You cannot exchange a call to one function with a call to another one with different name but identical signature.
- No overloading. You cannot provide two functions with identical names but different parameters. (This is not possible in ABAP OO too, by the way.)
- Variable encapsulation. Function groups can hide internal state in private variables.
- Method encapsulation. Function groups can hide internal methods ("form routines").
Like Jagger and Sandra Rossi suggest, think of a function group as a global abstract final class with static public/private members.