As a dabbler in the toolsets of differant programs, Freespace 2 is one of the easiest to mod, at the initial stage.
Doing simple custom missions, or modifying already created ships, is dead easy. Far easier then most modern games, where you have to jump multiple hoops to do anything interesting, esspecially with scripting.
But the moment you want to do anything complex and game changing, it gets harder, atleast compared to the gold standard of the Unreal Engine, where, without code acess, you can practically replace the entire game except the basic modules without touching the engine, just Uscript code.