SNES is about as hard to develop for as NES. It's more complex so there's more of an up-front learning curve. But SNES can easily do things that are considered advanced tricks on NES, so it kind of balances out.
I think the main reason people avoid the SNES though is because it's held to a higher standard. If you make an NES RPG, people might compare it to Final Fantasy. If you make an SNES RPG, people might compare it to Final Fantasy 6. It takes quite a bit of talent to meet the expectations people have for a game on the system, very difficult for one person to pull it off working on their own.
RPGs tend to be pretty big projects, if you can keep it extremely small in scope, maybe it's OK for a first project. But most people will advise starting with a smallest project possible and finishing it (like Pong or something). Your skills will grow over the course of a project, and you might find the code you wrote when you were inexperienced becomes a hindrance later. Which can lead to rewrites, stagnation, frustration, etc.. That's part of why people recommend smaller steps that are easier to complete, the experience is valuable later on.
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