In a similar vein to what TANSL said, a foil is a foil is a foil is a foil. Fin keels are foils. Daggerboards are foils, Spade rudders are foils, and lifting foils are foils. From a really basic, zeroth order perspective, there isn't any difference in their accommodation into a hull. They are all a matter of applying a bending moment to a hull (the bending moment usually drives the structural engineering). Also, the wetted surface of a planing hull is a foil. So it's just a matter of selection and optimization.
Ballast keels are the really clever ones and the ones able to exploit weaker materials. The moment at the keel root due to the hydrodynamic sideforce component is mitigated by the moment from the keel ballast acting in the opposite direction when the boat is heeled. This lets you build higher aspect keels than you otherwise could. So if I'm sailing hard-pressed at 7 knots and 14 degrees heel, the 14sqft keel has about a ton of sideforce and 3000 ft-pounds of torque about the root. The net ballast is about 7500 pounds at 1.8 feet from the root, so about 3200 ft-pounds the other way - nice from a structural standpoint.
Spade rudders don't have this advantage, but they don't need to be as big either. Good ones are very expensive components. Most production boats don't have good ones, and they are often smaller than they should be for the expected performance of the boat. High-strength composites allow the section thickness to be smaller for a given bending moment. But the gains are quite modest for the costs involved. Composite spade rudders began to appear in the early '60s on boats such as the Cal 40. They were viewed with great skepticism at the time and considered to be ill-suited for normal sailors (dangerous, race-only, and demanding to sail).
Daggerboards are intermediate between keels and lifting foils. They came of age with modern composite construction on boats like the IMOCAs. They can exploit asymmetry, and can vary span and area to match the sailing angle, but they need a deployment mechanism, bearings, and a box in the hull. They are normally only lightly ballasted and transfer all of the bending moment to the hull bearings. These modern, performance daggerboards haven't caught on either. You need to pair them with a canting keel to get the most out of them. And those are still regarded as dangerous, race-only, and demanding to sail.
Modern foils like on the recent generation of IMOCAs earn their keep by being sort of Jack-of-all-trade devices. They provide side force mainly as compression at the foil bearings. This reduces the moment arm of the aero-hydro sideforce couple, reducing the total RM needed to counter a given sail pressure. This lets you trade a bunch cheap, high RM lead for a bunch of expensive, low RM carbon composites to build the foil housings and mechanisms without increasing weight (almost, the most recent IMOCAs did gain some weight if I recall). Then you need to exploit them for gains somehow. The foil has two primary drag components - friction and induced drag. Ideally, they have nearly the same value. You need to reduce the hull drag by more than the foil adds. If you do this by attacking wetted area, you need to reduce the hull wetted area by twice the total wetted area of the foil. But it may be possible to reduce other aspects of hull drag, particularly in really fast planing sailboats.
So the real issues are systems integration and how demanding these systems are on the sailors. Controllability is improving rapidly, but is nowhere near plug and play. You have to trim the foils just like you trim the sails, it's harder to see what you are doing, and the controls are under water. You have to deal with the dangerous, race only, and demanding perception.
You have to deal with the costs. It is often more cost effective to optimize the mature tech as opposed to adopt a cheap version of some new tech. For a rudder system, I would rather have a skeg hung rudder and a good autopilot connected to a nav package with routing software, than a spade rudder alone, and the costs are probably comparable. So by the time you actually consider foils, you already have all the other go-fast gear since the economic returns on it are better. Foils would only be revolutionary (or disruptive) if they became cheap enough that things worked the other way around - you would rather have a cheap version of the foils rather than the full kit and caboodle development of today's sport keelboat. This could happen in some small corner of the sailboat design space fairly soon, but I don't see it happening to typical cruisers. Most of them would rather someone invent a better anchor.
