Dave Coffinmaker pretty much covered them all. I personally detest transfer bars primarily because of what they do to the trigger, that is it forces the trigger to sit out toward the center of the trigger guard. That and Ruger chooses to put a silver trigger in a blue revolver thereby giving a Baikal-esque look. I've always said of Ruger's- They're built like tanks, they look like tanks, and they feel like tanks. I own three Ruger's, so I have the right to my opinion on the matter.
My personal favorite is the pivoting hammer block which Uberti used until the advent of the retracting firing pin (RFP). It is actuated when the hammer is pulled back to the first notch. In my opinion it's very unobtrusive, and although I've yet to drop a revolver and test it, I'm sure it's quite effective.
I still say the standard safety notch is more effective than given credit. I've said it before, and I'll say it again: For more than a century and a half hunters, soldiers and shooters in general have galivanted about the woods, plains and battlefields carrying all manner of rifles and shotguns with caps on the nipples/rounds chambered and their hammers pulled back to the safety notch. Stop and think for a minute how many long guns utilize the half cock safety; all manner of muzzle loaders, the Henry, 1866, 1873, 1876, 1886, 1892, 1894, the Colt Lightning, '97 Winchester and many more. Yet for some reason many of the same people consider it fool-hardy to do the same with a revolver.
I've heard every excuse imaginable as to why it's OK with an 8 lb. long gun, but not a 2 1/2 lb. revolver. I think my favorite is that the safety notches on the hammers of long guns are "stronger". A couple of weeks ago I was perusing eBay and Gunbroker looking for a new hammer for an old '94 Winchester. I kid you not, there were TWO '94 Winchester hammers with broken safety notches on Gunbroker.
The most effective safety lies squarely between your ears. I've always felt that when carrying a revolver, a fella should always have in the back of his mind where it is relative to ones activity. Last week I was sorting through some cattle, climbing in and out of the lot, in and out of the loading chute, loading calves, etc. As always a Uberti SA was on my hip and as always, where that revolver was never left my mind.
Cholla