Gawd...I just did something dumb and deleted my answer.
There is no reason why a yacht with a drop keel shouldn't perform as well as a traditionally configured yacht, providing the whole thing has been put together properly and is sailed in conditions appropriate to the particular craft. There are a few different forms to consider. Plate only, plate and stub keel, dagger, fin or long plate etc.
Many trailable yachts have no stub keel and have only a drop plate for ballast and directional stability. Larger, more substantial boats will have a stub keel which may account for anywhere between 40 and 70% of the boats draft and ballast. This means the boat will track well even with the keel lifted, whereas those with no stub, and the keel raised, will sail sideways in any sort of breeze.
Apart from giving directional stability, the drop keel provides ballast and it is very important, especially in a larger yacht, to ensure the ballast ratio is true to the architect's design...and to determine just what was the intended use of the boat. It is worthwhile here to note that some Perini Navi boats of up to 150' in length utilise drop keels and can sail safely on any ocean in the world. Boats with no stub keel, generally rely on the raised drop plate to be housed in a box beneath the saloon table. This robs the inside of some space. Also, in these boats headroom may be limited in an attempt by the designer to increase the effect of the ballast in the plate.
It is important to know the AVS (Angle of Vanishing Stability) of the boat with the keel up and with it lowered. The position and weight of the ballast contained in the drop plate will have a major effect on the AVS curve..consequently upon your comfort and safety. (With the plate raised the arm of the pendulum is shorter...think of how a metronome works).
The manner in which the drop plate is raised and lowered is also important. It is manual or electric winch, gearbox or blocks and cable, or hydraulic...or is it simply a matter of heave-ho and burst a blood vessel. Is the plate hinged on a throughbolt at the leading edge, or is it lowered on a parallelogram, swing arm arrangement.
The keel must be lockable in both raised and lowered positions, not just rely on the weight of the plate. (Without a lock, the momentum of the plate could smash through the box in the event of a knock down, and could split the hull). The one absolute in all of this is that drop keels, especially in smaller boats, place mechanical workings in salt water, and allow the entry of abrasives. Regular and detailed maintenance inspections are mandatory.
Take a look at the boat out of the water and see the keel lift and lower. It should operate smoothly, without clunks or bangs, and it should have no more than the slightest movement in either the lateral/horizontal, or vertical planes.
Finally have the boat assessed by a surveyor who is familiar with drop keels. A long time ago I sailed a 23' Endeavour with a stub keel and drop plate. It was a marvellous inshore yacht with a keel up draft of 30". One time when pulling it out of the water, the drop plate fell off. The throughbolt in the upper leading edge of the keel had worn through and was as thin as wire. I was an inexperienced sailor then and had yet to learn the value of regular maintenance. I eventually sailed that boat offshore and was sad to see it go when I upgraded.
I hope this is of some assistance.
"if at first you don't succeed....Redefine success"!