I actually did rethread one to 10mm once because the customer removed them himself.

Otherwise I haven't had any 3/8 ARP bolts break as of yet.

I guess it all depends on how they break off, if flush in the hole they thread right out but if the break is crooked then you're gonna have an issue.

I've had to use the mill on a few because there was no way of removing the upper half of set screw.