It's really hard to say. It probably depends on the amount of inbreeding and any breeder could do that.
I just got a nice VT from my local LPS. They are pretty good and try hard. But I went in a week later and found them treating their latest delivery for fin rot. They say it happened after I got mine, but my boy has started showing some shredding. I started treating him yesterday with Maracyn Plus. I will do a couple of treatments with that. If that doesn't get it, I will do something more aggressive.
It is just as possible that a fish can come from an expensive line and develop something. The stress gets their immune system down. That is why it is always advised that new fish be quarantined.
Knowing your breeder helps, but it is not a guarantee. Fish are more sensitive than dogs and cats. It is a hobby full of heartache and exaltation. I have quit Bettas many times. But here I am still.