Do not base your buying decision on how expensive the hearing aids are. Costco’s hearing aids cost less than half of what you would normally pay for a similar aid from a hearing aid specialist.
The main disadvantage or advantage, depending on your viewpoint, is only Costco can adjust your hearing aids, unless you decide to become a do it yourselfer.
The variety of hearing aids at Costco might seem limited, but they know what will be most appropriate for your loss and features you desire. They really don’t want to sell you an inappropriate aid because it requires more fitting and adjustment time, which is counter productive. Costco’s marketing model is to get you the proper hearing based on your needs in the shortest time possible.