Donwyn, can you elaborate?
I understand Donwyn so, that you (or one) should not look at Schuberts music (instrumental music) as consisting only of melodi and chordal accompaniment, but you should look at what happens in the inner parts, Donwin in this way suggesting the presence of some polyphonic writing. I can not say, that I think Schuberts music immediately invites to such a perspective, or it may be the attitude of most performers, which make me think so. But the idea is thought-provoking, why I take it ad notam.
Well, this is from the hip, so to speak, as I lack anything in the way of advanced musical training (had some college courses, though)...but I'll hazard a sort of thumbnail summary...
I wouldn't necessarily call Schubert 'polyphonic', but I'd go out on a limb and say there are kernels of such a thing in the music. It's more a 'multiplicity' of sorts than anything, taking on the image of polyphony what with all the musical ideas barbing at each other and invading each other's space. It often reminds me of Stravinsky in that there isn't really a sense of a "Beethovenian Building Block" type of structure - just heaps of notes arranged strategically to provide firm and definite shapes. Large, sweeping gestures (themes), while present, need not carry as much import.
It's here where I feel Schubert loses his 'classicist' identity and looks more to the future. I think of Berlioz, and perhaps Liszt and Wagner, and on into the twentieth century. Classicist structures as a whole obviously dissolved over time and I believe Schubert was one of the first to make good use of such a change.
I've long given up looking for classicist roadsigns in Schubert. There's no denying that earlier in his career he held to earlier conventions but there was a gradual transition from about midpoint in his career that edged closer and closer to a full-on experimental format. Overlap occurred of course with conventional methods still vying for space alongside all the tinkering but the watershed moment came at about the time of the Wanderer Fantasy. From that moment on Schubert never looked back.
Cast in a structure fully reminiscent of the romantic era - one continuous movement - the Wanderer Fantasy was the deepest Schubert had gone yet into new territory. Full of pathos, it is, and dripping with a new kind of Schubertian emotion. No more pretty tunes, just good solid emotionalism - a la romantic.
From there the floodgates open wide. Listening to the opening movement of the B flat piano sonata (his last) it's impossible to think of Schubert as mere Tunemeister. The "New Multiplicity" has taken hold. There's a precipitous bent to that movement and it's a testament to his technical prowess that it succeeds so well. Can't chalk that one up to mere melody...
Anyway, 'daunting' is the way I define Schubert from about midpoint in his career. Little of the 'simple' or 'tuneful' rears its head by this point (think early Beatles and late Beatles

). With this new musical paradigm comes added complexity. But in my experience, on record anyway, it's an uncommon thing to encounter a performer who's willing to sacrifice the 'tunefulness' to get to the more decadent side of the music. Fortunately there are those who DO take a liking to this side of Schubert and it's to them I warm to most.
None of this of course is really an adequate explanation of what I mean by 'inner complexity' in the music. All I can say is that Schubert to my ears really took a liking to tinkering with layers. Unfortunately his 'tunefulness' carries so much weight that that's what gets the lions share of exposure.
In closing, I offer this quote from Liszt (about Schubert), saying in just a few words what I've failed to say in all this rambling:
"Such is the spell of of your emotional world that it very nearly blinds us to the greatness of your craftsmanship." -- Franz Liszt
[Forgive any typos and such...it's late...]