I don't think it's an "overambitious nature of HP fanfiction" but rather overambitious authors. Lots of people will get great ideas, and lots of people can write intriguing and interesting stories. But being able to carry a fic to it's conclusion, to stamp that "THE END" despite not using all your ideas or perhaps not quite nailing all the scenes and bits you wanted, that's sadly a rare talent.

First time authors don't often grasp how much harder the last few chapters are to write than the first few. It's also why everyone's jumping to Joe's defense, and rightly so. He's shown he can finish an epic storyline. Most of those others on your list I'm not sure they've got it. Jon3776 in particular writes some great starts but seems to lose interest and starts up other new fics.

That and in my experience, unless you're willing to cut lots of bits out, your story's going to be longer than you expect. (My longest I started thinking would be 40-50K, then maybe 100K and it finished at 290K. Sigh. My next novel length I planned around 100K.... it was 240K.)

 

This.

Is me.

Personally, I doubt that I've gotten in over my head with my plot - yet, at least. However, I doubt have the skill to pull off what I have/had planned as great as it could be. Either way, real life demands are killing my attempts (it has been since December, I think, since I updated...). I've followed half a dozen stories that have died for similar reasons - college/university, work, or having just gotten in over their heads with their plotlines; too many ideas and trying to cram them all in/writing themselves into a corner.

So I'm going to agree with Pers and Tinn on this.

 

Opposite for me. All the fun stuff happens nearer to the end, so the first chapters feel somewhat awkward to write.

Ambition is a tricky one though, but I can't honestly say that I've had a massive ambition problem... yet.