The opposite of logical is illogical, not "unlogical".
The 20th century can't be generalized to the degree people are doing here. Rachmaninov's very tonal music was written in the 20th century primarily. So was plenty of other accessible tonal music, like Barber's Adagio for Strings. Scriabin's concerto is another good example. So is Prokofiev's last sonata and his 5th, 6th, and 7th symphonies. Some composers, like Ornstein, composed tonal music in the 1970s and beyond, although many of them, like Ornstein, worked primarily with more recent systems.
Moving away from conventional tonality, music can become too cryptic for many listeners. Some composers, like Schoenberg, bore me, but others, like Roslavets, thrill me. I love the first movement of Tcherepnin's 5th concerto, and have even come to tolerate Rautavaara's 2nd. One problem many mid-late 20th century composers have, at least in the eyes of many listeners, is that their music is too bombastic. Stravinsky's Le Sacre is tonal enough and varied enough, although it's brutal to listen to. Later music, such as Rautavaara's concertos and Tveitt's later concertos, offer a "wall of sound" with a flurry of notes and noise blasts from the orchestra. This sort of music doesn't appeal to everyone. But, the tonal composers who can be tepid, like Handel, don't appeal to everyone, either. Handel, as with most other composers, wrote pieces that I enjoy and pieces I can live without. With "modern" composers, the difference between their best work and their lesser work is often quite a bit larger, I think, because of greater creative freedom as well as technology advances. Glass' Music in Twelve Parts is abhorrent to me, but I enjoy his Hours soundtrack.
Composers shouldn't hide behind flash or technique. They couldn't in the 19th century and they can't today. There may be a lot of less-than-ideal music in the 20th century, but if one listens to some of the "Romantic Piano Concerto" discs from Hyperion or Vox, one sees that there was a lot of mediocre music by forgotten composers written prior to the 20th.
As for Messiaen, his Turangalia symphony is good (there are two versions; the later version is heavily cut), but I think his Quartet for the End of Time is rubbish.