There are several types of water pipes. ABS which is a plastic, galvanized which are old, copper, and now there are new poly-type systems. All are functional when installed and maintained. All have a life span.
ABS pipes (like other pipes) can be prone to failure. It can be due to improper joint seals, freezing, and/or unusual movement. Such as on mechanical equipment, like a garbage disposal and washing machines. It may not be a big deal. Sellers are required to disclose everything and this is for your benefit to inspect to your own satisfaction. Leave it up to the professionals and go from there. Disclosures and reports can be daunting, but do rely on the inspectors to help you sort through what are big items and what may be just minor maintenance. Good luck and hopefully it will not be a big deal and you will have a new home.
If the builder goes belly-up in this marketplace good luck in getting the issues resolved.
You say you saw droplets of water beneath an area that the owner claimed had been fixed? If it'd been fixed, you should be seeing water.
Also keep in mind that water leaks are sometimes difficult to trace. Water will sometimes seep between floors, then run along joists, outside of pipes, or similar for feet or yards before dropping down onto the ceiling of the floor below. So, just because there's evidence in a ceiling of a water leak doesn't necessarily mean that the leak originated there. Thus, replacing a seal above evidence of a water leak doesn't mean the problem's been fixed.
Ask your agent (uh oh, you do have one, don't you?) for his or her advice. At the very least, you'd want any and all repairs done at the seller's cost, and to be approved by your experts (a master plumber) before buying the property; purchase should be contingent on satisfactory repairs. Then, just to be safe, your agent might suggest that the seller put an additional amount aside in escrow to be used in case of further problems.