In church all things should be to worship God and edification. Is it glorifying God? Is it edifying others? Some churches I have been in that play music so loud it hurts ears is NOT edifying and they do not discern the Body of the Lord. Perhaps 5 people in your church like heavy metal, 95 don't. If you insist on playing a certain genre in church you will bring division, as brothers who are against it will leave. For this reason spiritual music is meant to have no genre at all..the words are most important, but this does not mean the genre does not matter. Church music should have no genre from the world. They should be simple melodies inspired by the Holy Spirit. This is what many of the melodies for the old and faithful hymns are.. inspired melodies from men and women of God. Can we say the same about heavy metal and others?
There is nothing wrong with any musical instrument or genre in and of itself. But Christians are not free to decide what style of music they should play in church. It is God's church not ours. Abel brought God the right kind of sacrifice that pleased Him, Cain brought the sacrifice he thought was convenient, good and right, but God rejected it. Cain did not obey what God instructed Him.
This is the irony I suppose - we consider a suitable genre of music for different occasions. For birthdays we play birthday music, for kids parties kids music, funerals and weddings a certain kind of music is played. But when it comes to church, you would accept any and all kinds of music as long as the "words are right"...lol.. But a song cannot be separated into words and music. It is both words and music. Both will influence your mind and emotions as much as the other. Try using heavy metal to calm a baby, see if it works. Try singing a love song in heavy metal.. Try being angry with someone using a classical string quartet. It doesn't work. If the words are spiritual then the music should be spiritual too. Heavy metal, R&B, Rap , is not spiritual. It is not spiritual because it is popular in the world of flesh. So.. anything popular in the world is likely rejected by God.
The genre should fit the occasion. Mostly we would not dare to play heavy metal at someone's funeral. Yet you would play it in the house of God? Perhaps you have not understand what church is..church is the gathering of believers together and in which God's presence dwells in a corporate multipled way..church is more important than a funeral so we should consider the style of music even more carefully. Considering the genre of music to play in church is like considering what clothes to wear to church. There are some clothes you would not wear to church (e.g. a bikini).. Similarly there are some music styles that have no place in church. Or do you not know that God is a God of order and not disorder? Certain genres of music are disorderly which is especially true of "screamo" music, so they have no place in a church of the God of order.
Frankly the origins of many genres are from the world, the popular worldly culture of our time. The church is not of the world. It is in the heavenly realms, therefore church should have heavenly music. The Lord would say 2 Cor 6:17 "Therefore come out from them and be separate, says the Lord. Touch no unclean thing, and I will receive you." Why take an idol from the world and incorporate it into church? The church is spiritual so the music must also be spiritual. If you cannot imagine the music being played in heaven, then do not consider playing it in church. Church includes singing spiritual songs, hymns and psalms to the Lord (Eph 5:19). Key words there is "spiritual", and "to the Lord". Since we are singing to the Lord, the genre should be something which pleases Him, and it should be spiritual not fleshly.. it should not stir up our flesh or distract from the Lord. Certain genres are associated with worldly things. Heavy metal with anger, R&B with lust.. consider the majority of songs that these genres apply to. Consider if they are of the Lord or not. If they are not of the Lord then do not bring parts of these songs (the genre) into the church. If you are using music to bring people to your church, don't think they are coming for the gospel. The gospel has failed so we will try popular music instead.