Twin Keel vs Bulb Keel
Generally bilge keels (the traditional US name for twin keels)have a couple major disadvantages. They give up a lot of performance and they are really a bear to unstick when you run one aground.
In most cases bilge keels have significantly greater wetted surface and frontal area than a fin keel of similar lift characteristics. This means significantly more drag. This negatively affects light air performance where the drag really can hurt performance. It hurts windward ability very dramatically in all wind ranges, and hurts heavy air performance where the greater resistance means that the boat can''t disipate the energy of a gust by accelerating and so heels more and also must carry more sail area to keep going in heavier conditions.
In sailing ratings between identical models except one has a fin and one has a bilge keel, the difference in rating is often as high as 30 seconds a mile which is a very significant difference, (all things being generally equal this is roughly the difference in performance between a 28 and a 35 footer).
Relative motion comfort and seaworthiness is widely debated and frankly depends on the specifics of the design in question. That said, bilge keel boats when of near equal weight generally have a higher VCG which would tend to suggest less stability and a less comfortable ride. This is sometimes offset by greater dampening created by the greater surface area of the bilge keels and by carrying more ballast.
Of course the deal breaker for me is the difficulty in ''unsticking'' one. When I lived in Florida I was a sailing instructor in a fleet of boats that includes both fin keel and bilge keel versions of the otherwise identically same English boat model. The performance differences were quite obvious to those of us who spent time jumping back and forth between the two keel types. But the real issue was what to do when you ran aground. Even though these were pretty small boats, when they ran aground they were planted. You could not heel them out. You could not pivit them or fishtail them out. You were aground and you had to hope this was not high tide.