Many Spanish friends regularly remark that most rock and pop music sounds far better in English than in Spanish. Are they right? If so, why? Leaving aside certain cultural inferiority complexes, there are important metrical keys to understanding the reasons.
A decent place to start might be translated versions of famous songs, such as hits by The Beatles. Their work does tend to be dreadful when sung in Spanish, even when the lyrics aren't clunkily reworked. Let's take the example of Yesterday and look at how it works...
"...now it looks as though they're here to stay..." That's written in trochees!
"...oh I believe in yesterday..." That's written in iambs!
In other words, just as stressed poetry doesn't work well in Spanish, so imposing Spanish lyrics on a song that originally used iambs and trochees is never going to sound natural. In the same way, translated nursery rhymes are also extremely dodgy (e.g. Old MacDonald had a farm is a series of trochees in itself!). Here's a link to a typically grim Spanish-language performance of Yesterday:
Nevertheless, I do believe it's easy to get too simplistic about this. Many Spanish groups have successfully fused elements of flamenco or rumba, etc, to their pop or rock, creating gorgeous lyrics that face no problems at all. Trouble mainly flares up when songs written for English lyrics are shoved into Spanish.
This is another example of translator..traitor and shows once more that it's impossible to reflect accurately the poetry or lyrics of English in Spanish and vice versa.