I love the Brahms Sonatas too - I reckon they're excellent works.
And he's considered one of the Top 3 Lieder composers (with Schubert and Schumann).
And if I asked to you list the Top
10 lieder composers, well, you'd be pretty screwed. You wouldn't even get as far as 7.
I'm just very opposed to making a God of a composer (or a performer, or any human being for that matter). Sometimes I think Brahms could have spit on staff paper, called it an intermezzo, and you guys would consider it another work of genius just because it has J. Brahms at the top of the page. When I judge a piece of music, I judge the music itself, and nothing else. The likelihood of one person composing hundreds of "perfect" pieces of music just doesn't fly for me.
So, yes, I'm saying it: not of all Brahms' lieder are great. A few are just I-IV-V-I, if not a tad more complex. Hand me any mushy German poem and I'll do that. Of course Brahms was a great composer and one of the best lieder composers, but don't just hang on that.