Boulez' Romantic and post-Romantic interpretations are, to my mind, roughly in line with his more-modern repertoire. There is a great emphasis on rhythmic articulation, clarity of line, orchestral transparency, precision, and architecture. He deemphasizes some of the overt emotional content, which is to say that he tends to go through the work, rarely pausing to linger or broaden his tempo to impress the point. He is very precise, usually toward the fast side, and very concerned with fidelity to the score.
His Wagner (Der Ring des Nibelungen and Parsifal) is criticized for being too fast and too light, if that gives you some idea.
His Mahler cycle has been very successful where the emotional content is not necessarily front and center (i.e., except M2, M8, and Das Lied von der Erde). I would say that his instrumental works are better than his vocal ones, but his M3 and M4 were pretty darned good. He was at, in my opinion, his best in M5, M6, M7, and M9, where Mahler's modernism becomes increasingly apparent.
I don't really agree that Boulez does away with the emotional content of Mahler, I think that's nearly impossible. Mahler was the height of romanticism crossing over into the 20th century. Take the first movement of Boulez' M6: very weighty, slow and crushing. The climax of the 3rd movement (on his recording) Andante is wonderful and full of passion IMO. I think his M6 is one of the best recordings in his cycle.
I've tried to pigeon-hole Boulez too, but he usually comes up surprising me in some way just when I thought I had him nailed down. Naturally, I don't like everything Boulez does in Mahler - I really dislike his M9 and I thought the 2nd and 3rd movements in his M2 were a bit too quick there.
True, Boulez is no Bernstein, which is good. I like Bernstein's recordings, but there should only be one Bernstein. I think Boulez brings Mahler's music and the emotional qualities across in a different but valid way.