i cannot predict or assure anyone that a seed bead is not going to have a crack as well as i cannot predict or assure anyone that buys a ceramic tipped or ceramic tube bobbin holder also wont be cracked. i have absolutely no control of what comes out of the company that makes them.
i also cannot predict or assure anyone of the quality of all other fly tying tools. some tyers have used metal tube bobbin holders for decades without a single problem. i also cannot predict or assure anyone of the quality between a $2.95 bobbin holder vs a $19.95 bobbin holder with metal tubes.
if it were me, i would delegate any problematic bobbin holder to the waste bin or start using it to hold various types of wire (lead, lead free, copper etc). then i'd search the different bobbin holder manufacturers for the least expensive ceramic tipped ones and replace all of my metal tube bobbin holders or bite the bullet and get the ceramic tube bobbin holders (tiemco).
i'm not trying to be argumentative here.
i supplied a simple solution to a common problem that over time can/will occur with some metal tube bobbin holders. as i pointed out above, there are solutions to the problem and in a price range to suit everyone's wallet.