I think you're wrong.
Now I understand that you, and many others, even poets I know personally, don't like Neruda. And I understand he is lauded by many, including many who like poetry in general but don't love it. Perhaps this irks you in the way that widespread reverence for Charles Bukowski irks me. And indeed we have to put up with all the coffee shop open-mic night assholes these poets have left in their wake.
Yet I disagree that Neruda's poetry has a "naive, adolescent feel" about it. You've admitted that this should not be attributed to his free verse. Okay. What accounts for this "feel," then? I'm left to suspect you're just being contrarian. Okay. I do that on this site all the time.
But leave it that. Don't make haphazard comparisons to a random smorgasbord of poets, and intimate that depth of subject is an issue. First of all, not only were Keats and Neruda writing in a different language, they were writing in a different century
. That tends to affect diction. Like, if Merwin (or Mitchell or whichever translation you've read) were an early 19th century British Romantic, his Neruda translations would be just a wee bit different.
Secondly, per "The kind of prose a Britney Spears sixteen year old schoolgirl who thinks she's an undiscovered genius writes in her diary." What kind of prose would that be exactly, maybe something like, "More happy love! more happy, happy love!" Sound familiar? My point being Keats and Neruda were both essentially Romantic poets, were they not? Neruda's "Ode to a Watch in the Night" is about, most obviously, time (it would be too embarrassing for me to deconstruct the poem here and pontificate like total jerkoff on the futility of physical love's struggle to transcend time). But what's a "bigger subject" than time? Doesn't Keats' "Ode on a Grecian Urn" address the exact same theme?
Or is Neruda's transgression that he doesn't indebt himself to Greek mythology? I'm being slightly facetious. I admit that, in this respect, there are more layers (density) to Keats' work, much more happening per line, than in Nerudas'. Then again, the simplicity of Neruda's lines leaves space for the reader to infuse them with his own complex associations regarding time, or whatever the subject may be.
At which point it becomes a matter of personal preference.