It is actually better to focus on just one vertical?

[Dnurz]

Alright so I have heard before that is better to focus just on one thing at the beginning and not go vertical to vertical / traffic source to traffic source, just pick up something, focus on it and try to be the better on that thing.... Is this right?

Because I do want to specialized on adult traffic and just that, it is better or it's not a big deal to go test