The easiest and most difficult characters I find to write are the two I love the most - Enjolras I find the most difficult, and Courfeyrac the easiest.
We have a desire to ground these figures in realism (as we understand it). As Hugo gives us enough potential biographical holes to fill in around his suggestions and statements regarding their personal histories and characters, this usually works providing the author has done their period research and has a reasonable grasp of characterisation...I can buy differently nuanced Prouvaires, for example, even if I prefer some to others.
Enjolras is different. I became involved in a political party before I was old enough to vote, worked actively in student politics throughout my final years of highschool and university, and my first job was as a political adviser. My current job still involves a good deal of work in this area, albeit from the perspective of a public servant in the arts community. I've known many young, passionate, charismatic individuals (some of whom have gone on to serve as MPs and even ministers in state and federal government). I've known variations on most of the ABC boys - several Courfeyracs, some Combeferres, Bahorels, Prouvaires etc. Not exact approximations, but enough to recognise them. I've never met an Enjolras - not in person, and not in any of the eras I've studied. De Valera doesn't work (even with his adherence to abstract ideas combined with more than a passing knowledge of real politick), any more than Michael Collins or Wolfe Tone or any of the pantheon of French revolutionary figures. There are elements there, yes, but none of them come close to Enjolras as Hugo writes him.
I can accept a well written Enjolrai firmly grounded as a character in a physical world and specific time period, but they do seem to largely exist as distinct from the character Hugo created. As for trying to climb inside his head...nope, I can't do it. I'll eventually re-write the few first person POV stuff I've done with him, and am dreading tackling some upcoming first person POV, even thought I have the outlines there. I can write charismatic, passionate leader (whether I do so successfully is up to others to determine!), I can try to tackle personal reserve, I can attempt lofty abstraction coupled with hard-headed realism, but I can't come close to what I love in Hugo's Enjolras. He exists so thoroughly in the realm of the ideal and symbolic...but as Marianne brought up in some of our meta discussion in Paris (and I wish I could recall her exact phrasing), he has feet just barely, barely touching the ground and linking him to the world his friends occupy.
I think you phrase the difficulties of writing him as a narrative voice perfectly, Marguerite. There are dangers in him drifting too far into the mundane (or, as in some specific instances I can think of, in a hostile writer turning him into a petty martinet who exists solely behind a facade of his own construction...something that jars in a very grating way with the Enjolras of the Brick). He is far easier to deal with from an observer's POV, as then the observer - usually one of his friends - can have moments of comprehension and insight (as his friends do, e.g. in the compassion with which Combeferre and Prouvaire regard him when he executes Le Cabuc), but can also acknowledge those moments when he seems obscure to us, his thought processes opaque (sometimes because he is thinking so far ahead of his colleagues, even in a practical sense).
Courfeyrac, on the other hand, I find a delight to write. Hugo is so right - he is a breed. I've known Courfeyracs, delighted in their company, and find their combination of warm generousity, occasional earthiness and passionate idealism that can surprise those who don't read their joie de vivre correctly and think it precludes serious thought and feeling.
"The principles of freedom, of humanity, and of justice, will finally triumph over falsehood, tyranny and delusion, and every principle inimical to the interests of the human race" - Edward Despard, 1803