Why are vias bad?
I think the main problem is: vias could occupy significant space from other components, thus a larger board is necessary.
On the first picture a TH vias allow us only four pads to be placed. But with a blind via or without a via we have place for six (or more if we have more rows) pads. A larger BGA component could be placed here this way. source
And at the end reduced size means reduced cost.
But to defend the vias a little:
There are cases when they are useful. For example at high power dissipation componenets thermal vias could be used to help dissipate the heat by leading it to large copper-pours.
All in all it is very application-specific and could have both advantages and disadvantages as well. It is up to you to find the balance.
I wouldn't say that vias are bad. They are not!
One useful way to use vias is to shield RF energy in a RF board, a technique called via stiching:
It is just one of the parameters you can use to tweak the autorouter. Via's add a little cost in drilling (even though this might not be explicitly shown on the bill), they take up space, and other things being equal it is better for a route to stay on the same layer.
I can imagine (but I am not sure) that a via is just a little bit less reliable than a simple copper trace.