Mercy ship has the capacity as large as 1,000 beds and capability of receiving about 200-patients per day. Expert surgeons can perform surgical procedures regardless of various sea conditions.

They are too slow, with a maximum speed of just over 17 knots. They are too large, and are not easily or quickly deployed or docked. Their size gives them a substantial radar signature that, combined with lack of maneuverability, makes them vulnerable to attack (which should not occur under the auspices of International Law). The Mercy class ships are much larger (in terms of medical capacity) than is needed for most military operations. 

While the ships are underway or in rough seas patients must be transported by helicopter. However, helicopter capacities are limited, as each ship has only one landing pad.
Patient movement within the ship is limited. Built as oil tankers, the original oil storage bulkheads were retained, but have no hatches, which means that patients must be brought up to the top deck in order to be moved from a lower compartment in one part of the ship to another.