Over-training is the biggest problem in volume training. Nobody can say exactly when over-training occurs.

As a consequence there are too many training plans. Everybody recommends a different training plan or exercise as well.
Only one thing is common: nobody can explain why their technique is the right one.
Sometimes you could assume, set numbers like 3-5 or 7 had something magic because they are used a lot in training plans.

Because of the big confusion resulting from this, the opinion of most of the readers is: everything works!. Frequent switching is the true key to success here.