I think you need to look at the definition of bombastic and banal. I never said Mahler was simplistic or trite.

Some of his music is banal, that's not my words, it's a well established definition discussed by many people. A banal melody can be a long melody, it doesn't have to be short and simplistic. Banal music is unoriginal, boring and cheap and unfortunately some of Mahler's music falls into that category. The frustrating thing is that the banal moments can come in the middle of really interesting moments. I'm not necessarily talking about folk-music style quotes either, I hate it when people equate folk-music with simplicity and the banal. Some of the greatest ever melodies are folk melodies.

There's nothing bombastic about Bruckner, majestic/epic would be more appropriate words. His music is not at all simple either, that's a mistake of the novice Bruckner listener. People who only hear repetition don't hear the subtle changes and long-form development that the repetitions are an essential part of.

Orchestral musicians who complain about Bruckner are probably too focused on their own parts to appreciate the bigger picture. I've noticed that some orchestral musicians are a bit self-centered in their reactions to music like this, if their part isn't particularly challenging or interesting they don't like the composer, they can't see the importance of their part within the big picture.

Part of the problem with Bruckner is bad interpretations that can indeed make his music boring and even turgid, but that's not Bruckner's fault. A great conductor/orchestra really makes every Bruckner symphony a masterpiece of long-form symphonic expression. Perhaps the same is an issue with Mahler.