Look at the red square of my blurry diagram below - Licks/Study. (Notice
on the diagram all the levels are in relation to what's hard or easy
TO YOU) It's important that you don't work on songs that are too
hard for you. A very few people may respond to the 'throw yourself
in the ocean to learn to swim' method, but most people need little
bites and chew well - for the fastest progress! That's what we all
want: to learn as quickly as possible. It's also important, as a learning
tool, that you are encouraged by your progress regularly, not staring
at the same sheet of music for 6 months.
I think maybe the most important reason for not choosing too difficult a song is that it consumes all your progress. You pour 300 hours work into a song that needs 1000 hours at your current level and if you'd given 10 hours work to 30 pieces they'd sound awesome and you would have grown much more. I've seriously seen people spend 2 or 3 years learning 4 or 5 pieces. THEY WERE GREAT PIECES, but I think the large majority of people would be much happier learning 30 easier pieces that all sounded great and developed their musicianship over those years, making each song easier and smoother and more exciting. I'm convinced people learn much more quickly that way: smaller but consistent progress on manageable pieces.
Hard pieces (hard to you, again) are great for inspiration (targets) and as learning tools (what did he do here and here?) and are a great place to learn cool and interesting licks and riffs (bits of the song) from amazing artists. But - when you're working on licks, still keep it at a manageable level too. Most people need to work on just 2 or 4 measures at a time (or in toto ), not slave over the entirety of 'Eruption'. At least not until its only somewhat above your ability. Remember, small bites and chew your food.
