1) If they go overdrawn it is what is called a deemed taxable distribution and it ceases to be an HSA as of Jan 1. Section 4975.
That makes it a pain in the rear to monitor and keep up with.
2) Also, you will notice they are sole owners only, no joint, so you'll have issues with other parties taking the debit card to pay for things.
3) You also must monitor contribution limits, the bank must stop any transaction that would cause them to go over the family limit for the current year.
4) Due to the nice tax advantages, lots of people horde money in them (which isn't wrong) but they can also reimburse themselves for other purchases that they have made using the HSA. That in and of itself makes layering a very real possibility using an HSA. They can buy stuff with a credit card, deposit cash into their HSA, payoff the credit card and then claim they were just getting "reimbursed" for earlier medical expenses they had to put on their CC.