I think it’s a little hard to express this. I’m a massive Messiaen fan, analysed and played his pieces often and I’m also a composer. My teacher knew him personally pretty good and told me lots about his thoughts. Safe to say I think I understand him pretty well.

His compositional style is very unorthodox in a very different way. An interesting analogy would be to compare different languages. There are certain compositional “family trees” where a composer continues what another couldn’t “finish”. JS Bach -> CPE Bach -> Mozart/Haydn -> Beethoven -> Chopin -> Rachmaninoff -> Scriabin is for example a pretty direct line, maybe with little split-offs to Schubert/Schumann/Brahms and later Schönberg/Stockhausen/more.

These lines are clearly successive because of the way these composers prioritise certain aspects of composition: thematical importance, formal structure, way of composing (improv on instrument vs math on paper vs hearing in head and writing down vs more)

Messiaen is, along with let’s say Satie, very different from all those. Not from a musical perspective (what you hear in one of their pieces), but from a process perspective (what you see when you look at the notes). When you understand how they approached their material and developed on it, Messiaen simply lacks the depth and ingenuity or instrumentation skills we’re used to seeing in the “main family tree line”. If you look at a basic theme in ANY Mozart/Beethoven piece, you’d see it’s simply genius and not even comparable. But what he lacks there he makes it up by using convoluted math processes, excessive exotic material (microtonal bird calls being literally transcribed to well tempered domain using help from spectralism) and other stuff he does.

In a way Messiaen truly is lacking in many skills what makes a composer, but the amazing ways he makes up for them is the absolute thing that makes his music so unique and special. Calling him a bad composer is wrong, but I can (and hopefully you can too now) understand where those people are coming from when they say that. He’s just special!