The problem with snakes that have had mites for a while (& you said his treatment has failed in the past) is that it increases the odds that one or more of the snakes
may come down with "who knows what disease" that was spread from the mites...and even when you get rid of the mites, you won't know for sure they're healthy
for quite some time to come. There's always the risk that you might accidentally transport one or more mites (or eggs) out of the quarantine room also.
Whether or not it's worth the risks & hassles to you, only you can decide...the answer may be different for others, since each of us has our own circumstances to
consider. This guy with the 5 BPs obviously didn't do a good job of eradicating the mites...you might want to ask him what he used, & you might have to toss a
coin as to whether or not to proceed & add them to your household. Good luck either way...I feel sorry for the snakes, it's not their fault.