OK, first and foremost the main problem with the crap vessel set is the cheap quality plastic risers. These were letting virtually untreated water past the rubber O ring and off towards the glass. This can make the "bad resin" seem random because if the warped bit is pushed up far enough past the O ring it's fine. How far it's seated is variable depending on how full you fill the resin and how big the bit at the bottom of the riser is.
Next up is the baskets. The crap ones (below), simply slot in, sometimes with a spot of glue to stop them falling out. If it falls out resin gets wedged in the O ring and lets untreated water past. If it's glued in then how do you replace the perishable O ring?
Below is a good one, the basket screws in. There are a few variations on this but in general some fixing mechanism is needed.
Finally is the bit at the bottom of the riser (sorry don't know what it's called). On the cheap ones it's too big and makes it more difficult than necessary to refill the resin.