Are resistance Bands as good as weights for building muscles?
It depends on the kind of exercises that you're doing.
The main difference between bands and weights is that the resistance increases when you pull the bands whereas it stays the same for weights/cables. The bands offer zero tension at the point of maximum extension and highest tension at the point it max contraction.
This might be a good thing for some particular exercises but for some others like a shoulder fly or bicep curl, heavy bands might strain your tendons a lot more just because your muscle is pulled the maximum at the highest point of eccentric motion.
Speaking from experience, you should avoid bands for targeting smaller muscle groups. And being someone who has been whipped across the chest with a heavy band(took 3 years for that scar to disappear) , I would suggest you to stay the hell away from bands at any cost.
The only resistance bands I would recommend are the loop bands which are used by athletes.