All depends on the boat. I used legs quite bit on a 26ft long keeler and it was very sucsessful. Haven't tried them on anything bigger but I suspect they might get unwealdy. Provided the pad is the right size I have not had problems in softish ground as the legs and keel sink the same amount but would not try it on something like essex mud! just let the keel sink in anyway. More of a problem is if the ground is uneven, I always try to settle bows to the bank for this reason but ideally you want clear water or to Know the area well, ie checked at last low water.
We had legs from the yacht leg company on our freedom 35-superb.
We used them to dry out freestanding or the outer leg only as backup when drying out against a wall-first time slighty heart in mouth but in practise thereafter solid as a rock and never any doubts-would willingly use them again. Mind you - our Freedom only drew 3'6'' keel up and had a long wide keel to settle on, would be interested to see what they would be like on a shorter fin keel.
My father's boat 38' LOA 9' beam had legs for 30 years with no problem. The length of the legs must be set such that you do not become 'leg bound' - ie the weight of the boat on both legs. The leg should just stop the boat heeling over.
One can always add a pad to lengthen them or remove one, depending on the bottom.
I think that in very thick mud legs would not be recommended.
Used them for 12 years on a small long keeled traditional boat, and never had a problem, but we always checked the bottom on a previous tide.
As has been said, they need to be a bit shorter than the draft, secured in the fore and aft direction (by warps to cleats or samson posts at each end of the boat) and ideallu shaped so that the lower ends are a little further apart than the boat's maximum beam.
They are a comfort and a saver of expense when laying up ashore, incidentally.
I know at least three boats that are kept on drying moorings using Yacht Legs (the product, not the generic objects) One is a long keeled motor sailer, moored fore and aft on sandy bottom in Angeley. There are two Sigma 33s on pile moorings in Ramsey, IOM, on mud. These are the fractional rig design with deep keels. I can't speak for the Sigmas but the Anglesey boat has been on legs for several years without problems. The owner advocates that the legs should be a good deal, say 4 - 6 inches, less than draught.
I also know a Starlight 35, now sold, which has a deep wing keel. Her owner frequently dried on legs.
My own experience is that I have owned Yacht Legs for 6 years or so. Have used them in a wide variety of bottoms, from concrete over winter to soft but not very deep mud. Perhaps 6 - 8 inches. Never had any trouble and see no reason why they would not work in very deep mud. Main problem is cleaning them and the decks if an early start is called for. My keel is shallow (long) fin, perhaps 5 or 6 ft long
I have yacht legs on my current shallow draft - but fin keel Jeanneau and have used them with complete confidence on sand. We used them previously on a fin keel (again shallow draft version -around 4'6" First 305) quite few times. Most memorably in the drying harbour on Lindisfarne. You get quite smug when those that can't dry out are ridding out in rough wind over tide anchorages!!
Beneteau supply very strong, (but a bit agricultural) telscopic alloy legs in two different sizes with all fittings to bolt to the hull. They are a lot cheaper than the Yacht Leg Comany ones - which are excellently engineered, but like everything you have to pay for it.
The Beneteau ones are extremly robust and should be available to order from the dealers.