It's harder for me cuz I'm making new characters, a new region, and everything. It's substantially easier if you're just changing a few things about the game, like trainer rosters or some of the events. So don't let what I've said discourage you from looking into it! =P
It can be, especially if you're working alone. A lot of the frustrations I'm going through right now is from working on art. Some of it I have difficulty doing, like sprites for the trainers in battles, since I either end up hating the final product or spend tooooons of time making them. It's agonizing. I would rather be scripting/mapping right now, since that stuff isn't very difficult for me to do.
The most stressful thing about all this is keeping myself going. Once I get enough momentum to do things, I feel like I can't take a break or I'll lose my drive. That's what it was like a few days ago, when I made all the remaining overworld sprites for all the characters in my hack + their animations. So glad that's done. Usually this ends in me overtaxing myself and taking long breaks. That'll probably happen soon lol ;x