1. Era of voice change: There are effects like voice simulation, relay effect & gadgets which can change the character of a voice, these effects can be superimposed & untrained ear would never know.
2. Editing: It is possible to edit and make a tough song easy.
3. Missing rhythm is nightmare: If one misses rhythm he will keep lagging behind the track ( Vinit's earlier performance of 'is pyar ko mai kya naam du: he missed the rhythm in antara it was taken care of by the live band).
4. Extra vocal effects: like singing at 2 notes higher, doubling the voice & addition of vocal chorus can be done in a track.
5.opening can of worms: Any such format opens can of worms with allegations like, a particular contestent is being favoured with higher bass effect, superior mixing etc.
In international competitions like X Factor there are strcit guidelines regarding how to make tracks.