I was a member of the Church of Christ for about 27 years. I served as a bible teacher, deacon, and ultimately as an elder. The c of C is an example of Christian fundamental extremism. Their members would deny this but they worship both the bible and the apostle Paul as well as Jesus and God, and pretty much in that order. They take bible inerrancy and literalism to such an extreme that it takes control of their lives to the point they become bible idolaters aka Bibliolatry.
It is their teaching that every word in the bible must be followed exactly as written. To be a Christian all commands, laws, rules, and even examples, which are interpreted as commands, must be adhered to precisely for a believer to retain their salvation. Sin, any sin, will cost the believer their soul, so sin, by any definition, must be repented of and forgiveness asked in order for the believer to have their soul restored. And this cycle is repeated each time a believer sins.
As others have noted the c of C does not allow instrumenal music in their buildings because nowhere in the NT is instrumental music specifically authorized. Therefore using instrumental music in worship goes beyond what is authorized in the bible. Since the bible doesn’t authorize it using instruments in worship would be a sin and that would put the offender's soul in jeopardy.
Another poster noted the International Churches of Christ. That group came out of the traditional Churches of Christ but they are a separate groups now they have no connection with each other. I would classify both the traditional Church of Christ and the International Church of Christ as dangerous cults with the notation the the ICC is by far the more problematic of the two.
Both groups shun disobedient or erring members. They call this practice dis-fellowshipping. Like all religious fundamental extremists they indoctrinate their members and this begins at birth. A child placed in their day care would be subjected to being indoctrinated in their religious beliefs.
Once you become a member of the Church of Christ it is difficult to become disassociated from them. Like all religious groups individual congregations (each congregation is autonomous the Church of Christ has no central authority) can range from liberal (by c of C standards) to hard core extremism, but their core beliefs and teaching remain pretty much the same. Congregation’s that disagree with one another often dis-fellowship each other. The level of legalism in the Church of Christ and the antics they engage in to insure doctrinal purity is almost comical.
Hope this helps.