SCUBA diving from sailboats is generally something that will require some special consideration. Most divers want to get from the dock to the dive site as quickly as possible. If speed or time is not a concern, then there are at least 5 other considerations.
Because sailboats are designed to take high seas their decks are higher than most other boats designed for the ocean. Diving from any vessel is best viewed as how easy is it to enter and exit from the water while wearing the required gear. Entering the water by jumping spread eagle or a back roll off any boat higher than 6 feet off the water would be considered dangerous. So, your entry needs to be from someplace closer than the side or transom of a standard sailboat. If you have to climb a ladder more than about six rungs, you'll have a greater risk of injury if you slip. Not to mention that following a long dive, many people are winded and tired, so this puts you at even more risk for an accident.
Scuba divers who do dive from sailboats typically will make their dive from the sailboat's dinghy. If this is an inflatable or small rigid boat with a ladder, it does make for entry and exit to be a lot safer and easier. The advantages of the dinghy are that you can anchor the sailboat in a sheltered area and use the faster, smaller more portable dinghy to ride to the dive spot. Once at the dive site, you still must anchor the dinghy and following the dive, return to the dinghy. The disadvantages are that the dinghy will most likely be unattended during your dive and you have to transfer the gear from the sailboat to the dinghy, which could be more risky in moderate seas.
Space on sailboats is always a premium. Except for larger catamarans designed for group day sailings, most sailboats have every nook and cranny taken up something important (sailing equipment). So, diving from a sailboat usually means keeping all your gear neatly confined to your equipment bag and stowed during the sail or motor to the dive site. Additionally, the dive tanks are always a hassle on a sailboat, even when there are racks to store the tanks in the sailboat's cockpit. Because of the tight quarters already, tanks make for a more difficult situation.
It's usually a small group setting when you're on a sailboat. Diving from a sailboat will naturally mean less number of divers. This is usually a good thing because crowds on dive boats are rarely seen as a positive. The one consideration for diving in small groups is the safety equipment on board and the qualifications of the staff in the event of an emergency. These are not usually the first questions you'll ask the boat charter people, but do not forget about safety equipment and procedures when booking a trip.
Is the scuba diving the first priority or do you want to do some sailing and take in a dive? If your priority is to do some sailing, maybe a full day with an hour or two out for a dive, then diving from a sailboat might be the perfect setting for you and your small group. If you have snorkelers in your group, you'll probably find this environment more accommodating to the snorkelers over the divers.
There's nothing finer than anchoring the sailboat in an isolated cove or beach area with some diving followed by a nice island meal served to the relaxing motion of the seas. Advantages or disadvantages, good or bad, suit your desires and experience diving at its best.