Where I diverge from fail forward (and this may be just because of my love of math and probability and how it interacts, and how I view it in the game, etc.) is that I dislike the tying of abstract thing to characters abilities.
In the example of the mace here is the way I see it playing out.
The DM does not know if the mace in the tower. With the fail forward example they will find it if it is there, and if it isn't there they will find the alternative path. So the difference between the 2 is not "did they search good enough" but is the thing they are looking for there.
My preference is (if you need to decide and the DM can't) just roll a die not tied to a skill (50/50).
One response to this was "we are tying it to "failure" not to the skill" but mathematically that is utter rubbish. You might mean that you don't care if it's tied to the skill, but it is easy to show that it is inversely proportional to your skill.
Skill will succeed on (p) so chance of failure is (1-p) so chance of mace not being there is (1-p)
As p goes up the chance of the mace being there goes down.
Amusingly (to me, at any rate), close acquaintance with probability and stochastic outcomes is a major reason why I see things quite differently in this respect. I'll see if I can illustrate why:
Consider a situation like the "finding the mace" case you cite. Imagine that we have a system akin to that you suggest, with one die roll - on a d10 modified by character skill - determining the thoroughness of the search carried out, and another roll - also on a d10 but unmodified - determining whether or not the mace is present to be found.
Now, consider further that we could devise quite easily a system that is
exactly mathematically equivalent to the system above, using a single percentile roll.
In this percentile system, the character skill has an influence on the outcome, but - considering where the original system to which our percentile system is exactly equivalent - the chance of the mace being present to be found is clearly not connected to the character's skill level.
In most cases in D&D, skill level is not so overwhelmingly important that it is determinative of success or failure. It has an influence on the outcome, but does not (usually) make it a foregone conclusion. In this circumstance, I see nothing whatever wrong with viewing skill rolls as being analogous to the above "percentile roll". In other words, it judges ("resolves") success or failure at reacing a desired end-point based on a constellation of potential failure modes or reasons. In fact, given the general ways in which feats of skill work in real life, I see this view of skill rolls as far more plausible from a "verisimilitude" point of view than the "you either bungled or you didn't" perspective.
As soon as a DM says to me "If you pass the roll you will find the mace, but if you fail the mace is not here and I will give you a clue to it's location" the "Schrodingerness" of the situation is staring me straight in the face.
I think you are vastly overstating the sharpness of the divide, here. Not every failure will be taken to indicate that the mace is not there to be found; all it means is that the characters' best shot at finding it has failed to uncover it. Given how destructively thorough most players can be in their imaginations when "pixel bitching" a room, I will grant that the chances that the mace is there and still undiscovered by a balls-out search is slim, but strictly it's just "unknown".
As an aside, this is one of the things I'm liking a great deal about Dungeon World, so far (still reading and digesting - not run it, yet). Part of the GM's Agenda given in DW is "Play to find out what happens". This discussion has made me realise some of the dimensions of this; it is really talking about the "author only what you have to" that [MENTION=177]Umbran[/MENTION] mentioned upthread. If the PCs fail to find the mace (that might or might not have been there), what do you as GM take as "known"? Simply that they failed to find the mace - no more than that. It might now plausibly turn up somewhere else, or it might not (whereas, had the PCs found it, it obviously could not turn up elsewhere unless they either took it there or lost it again). Each resolution in play sets the parameters within which future resolutions happen, and as a GM you can be as surprised by what happens as the players. Looking back, there have been elements of this in my GMing previously, but I had not consciously singled them out as something "fun" I wanted to increase and develop. That may be changing.