When it comes to a genre like war poetry, one often feels like one's missing something. Like there's something you can't quite access, as touching as the poem may be. It's not exactly that it's less universal than poetry in general; at least the great poems aren't. But then there comes the question of what it means to be "universal". If you mean "something everyone can sympathize with", even the less phenomenal but still heartfelt poetry of some of the less talented but still decent trench poets should count. If it means "something everyone can identify with," we have a much less broad range of work to consider, but we're still including the majority of poems; love poetry is probably the most obvious example of this (even though "universal" still doesn't include some readers--I remember that Shakespearean sonnets seemed little more than ridiculous rhyming exercises to my mind as a child). However, that last definition (if you're okay with calling such a loose suggestion a "definition") doesn't seem to include war poetry. No matter how good our imaginations, the concept of actually being (in the case of this war) in the trenches, of hearing the shelling, of seeing men dying everywhere..that's more than a little staggering. We can try to put ourselves somewhere near their proverbial shoes, but we'll never (hopefully) be in them, and to suggest otherwise even seems disrespectful. Unlike most poets, they're not struggling to express the an experience that is common (to some extent) to us all.
But to accuse the poetry of lacking universal appeal is absurd. While the experience evoked is not one common to all, what the poems do is make it accessible to all. In this genre, poetry takes on a greater communicative function than I think it usually does. Which isn't to say that no other poetry is communicative; it's just that in describing many things, a poet can to some extent rely on a similarity of experience...here the similarity is minimal, so the images used, the comparisons made, are often very much at odds with the lived experience. They're bringing the real world into conversation with the world of the trenches, and the result is jarring (the setting of sun as an image for the blood coming to a dying soldier's lips? awful!), but it has the desired effect of violently reorienting the imagination of the reader such that he is brought closer to the horror of the poet's experience.
Of course, the poems of WWI are very modernist in this way, even if they are a sub-genre of modernism and rarely as innovative in formal matters as the Modernist poems proper were. The superficial similarities of violent metaphor and the conception that "reality" is much darker than the era of Swinburne and his ilk wanted to admit certainly has its roots in the fact that both trench poets and Modernists were in a sense poets of WWI; both experiencing the tragic loss of a generation and the accompanying disillusionment and reflecting that fact in their poetry. Beyond the more superficial, however, it's interesting to note how the common method of jarring the sensibilities to achieve the desired effects on the readers imagination is rooted in very similar preoccupations for the two groups. The Modernists are primarily concerned with freeing poetry from the stultified verse forms and "poetic" language to which it had been restricted for the previous fifty years or so. They are difficult, though they are condemned as being elitist for it, not to make their poetry inaccessible to the common reader, but to make poetry back into something that could communicate.
Though the trench poets were not at all (at least not while in the trenches) on the forefront of literary innovation, they faced an analogous problem: the contemporary literary defaults were incapable of expressing what these poets needed to express; the solution? Use the old images in unexpected ways. Unsettle the reader. Don't give him what he expects. Another layer of complexity is added by the fact that these soldiers weren't writing with the intent of publishing, at least not that immediate intention. The subversion of expectations may then be seen as something of a psychological demand on the part of the writer; he needs to understand what he's seeing; these are the poetic tools at hand; how can he reshape them to express what he needs to?
So yes, the poetry is less universal in that the experience described is not a common one, but something that we can only access through a tremendous stretch of the imagination, and then only partially. But rather than being merely "something everyone can sympathize with" or being "something everyone can identify with," the poets' (differently motivated) use of the modernist technique of dislocating metaphor from the traditional assumptions about its meaning makes it something that everyone can, with a little extra work, empathize with. That is, they can feel with the poem instead of simply feeling bad for the poet, even if they can't (unlike in the case of the Modernist poem, note) identify with the experience it describes.
All in all, that capacity of the best trench poetry makes my 6 euro purchase of a little collection of Siegfried Sassoon, Wilfred Owen, Isaac Rosenberg and others well worth the money.
One of my favorite poems by Owen:
I saw his round mouth's crimson deepen as it fell,
Like a Sun, in his last deep hour;
Watched the magnificent recession of farewell,
Clouding, half gleam, half glower,
And a last splendour burn the heavens of his cheek.
And in his eyes
The cold stars lighting, very old and bleak,
In different skies.