I get frustrated by church worship music with a melody that is impossible for the congregation to sing, therefore rendering the worship song as PERFORMANCE by a WORSHIP BAND on the stage. That is not congregational singing. That is a concert.
There are many poorly written hymns as well -- with registers so high that only a coloratura soprano at The Met could sing. But generally speaking, the reason hymns are so easy for a congregation to sing, is because the melodies are EASY to sing. There are many modern hymns being written as well. Check out gettymusic.com.