The single greatest reason that I've been so harsh on Pokemon like Mega Sharpedo and Mega Camerupt is that despite the numerous perks that they bring to the table, I don't find them to be impressive enough to warrant using up my Mega slot on them most of the time. This was a present problem for many Mega Pokemon in XY, but I think that the recent influx of new Megas in ORAS has exacerbated the problem. To illustrate what I mean, the old viability ranking thread had 12 Mega Pokemon in the A-S Ranks. The new rankings including the new Megas have a total of 22 Mega Pokemon, nearly twice as what we had before (note: this isn't taking into account some of the rises and drops that have been discussed earlier in this thread). That means that every Mega Pokemon that was already lower on the totem pole to begin with now has to compete with many more Mega Pokemon that are just better overall, which is obviously going to have an impact on their viability (unless they happen to benefit from certain changes in the metagame).
Now, the obvious followup question is, "If the competition for that Mega slot affects the viability of these Mega Pokemon, then what does that say about the ones that are ranked at or near the top, such as Mega Salamence and Mega Charizard X?" Let me put it this way: when you hear me talk about things like Mega Camerupt and Mega Sharpedo, I often compare them to other Mega Pokemon that are better overall. The thing is, these other higher ranked Megas are the better Mega Pokemon that I'm talking about. They all offer something that makes them stand out not only among the regular Pokemon in the metagame, but among other Mega Pokemon as well. This can be anything from unique and useful traits that nothing else can boast to extreme talent in one particular area that makes them stand out (i.e. lots of Pokemon can Dragon Dance, but nothing sets up and sweeps quite as easily as Mega Salamence). These Pokemon aren't so much the ones that compete for the Mega slot with other, lower-ranked Megas, rather it's those lower ranked Megas that compete for the slot with these Pokemon, if that makes sense. All that said, the competition for the Mega slot is still there for these Pokemon, and it is somewhat awkward to weigh that competition more heavily for something like Mega Sharpedo than for something like Mega Salamence. That's why I believe some discussion on this factor would be a good idea and that we need to talk about what other factors should go into making this decision.
One of the things that I try to take into account besides the competition for a Mega slot is how much the Pokemon stands out on its own. Take, for instance, Mega Altaria. I'm fine with seeing it ranked as high as it is because of how unique it is. There are no other Fairy-type Dragon Dancers. There are no other Dragon / Fairy Pokemon in the game, period. While it may receive competition in some of its sub-roles (such as "Dragon Dance sweeper" or "bulky Fairy"), nothing offers the combination of unique typing, movepool, bulk, and offensive presence that Mega Altaria can bring to the table. Now compare this to Mega Sharpedo. The reason I keep comparing it to Mega Gyarados is because of how similar they are. Mega Gyarados has the same typing as Mega Sharpedo, a very similar offensive movepool, and a similar primary role as a physical sweeper. It also has a higher base Atk stat, far more bulk, a great boosting move in Dragon Dance, and the advantage of having a great normal form in addition to its Mega form. Now, Mega Sharpedo has advantages too: it has more speed, a stronger initial Crunch, and most importantly, the ability to basically guarantee a free Speed Boost with Protect. My problem is that I just don't see these perks as enough to stand out among other Mega Pokemon when another Mega (Gyarados, in this case) can do much of the same stuff with even more substantial perks of its own. It isn't just competing for a Mega slot; it's also competing with another Mega that is pretty similar in several ways, and thus it doesn't have those stand-out qualities that higher-ranked Megas like Altaria and Metagross have. Therefore, in my opinion, the cost of taking up a Mega slot weighs significantly more heavily against Mega Sharpedo than it does for many other Megas, and this applies to some of the other lower-ranked Megas as well. This is how I reconcile weighing the Mega competition more heavily against Sharpedo than for many other Megas in my mind.
Now, let me be clear about one thing: I don't think that Mega Sharpedo (or any of the other lower-ranked Megas I've mentioned in the past) are bad, nor that they may not be worth a solid rank in their own right. The problem is that I'm trying to approach this from a teambuilding standpoint. I could build a team around Mega Sharpedo or something and it would probably work quite nicely. However, when I look for a Mega Pokemon as a teambuilder, I'm looking for something that either stands out as extremely good at one particular job or something that can do something unique while plugging up other holes and/or offering unique team support. For example, I used to love running the standard tank Charizard X with a good bit more Speed and Attack investment because it could hit pretty hard, burn stuff, and check a lot of important Pokemon such as the Electrics, Mega Scizor, Mew, and Bisharp. I also liked DD Mega Gyarados because besides being a pretty reliable sweeper, it could also utilize its solid bulk, Intimidate, and cool typing in both forms to check several different offensive Pokemon such as Keldeo, Landorus, and again, Bisharp. Offensive Mega Venusaur was another one that myself and many others liked because it could both hit pretty hard and tank hits from a number of dangerous Pokemon (Azumarill being a big one), which was great for some offensive teams. Mega Lopunny is a new one that may not offer much defense, but its natural speed and perfect coverage between its two strong STABs along with the variety of utility it can have with its remaining two moveslots make it stand out as both a solid offensive Pokemon/check to many slower Pokemon and in some ways a team player as well. This tendency of mine to gravitate towards Megas that do more than just one particular job and claim that kind of versatility is one thing that makes me less inclined to pick Mega Sharpedo in teambuilding unless I just really wanted to use Mega Sharpedo. That's why I'm leaning to a lower rank with something like it; it can work when it needs to, but other Mega Pokemon can do their own primary jobs while offering other secondary duties as well, which I find Mega Sharpedo to have a harder time doing.
So to summarize, I think an important topic of discussion should be how we are going to weigh the cost of using up a Mega slot. This goes beyond just Mega Sharpedo and such: Mega Latios is one Mega that I'd argue is a bit better than regular Latios, and if it were ranked based on its own merits alone, it would arguably be worthy of something in the ballpark of A+ rank. However, it's currently ranked far lower than regular Latios is because, while it may be somewhat better than its normal form, the boosts aren't significant enough to warrant being used over another Mega form unless you just really don't have a need for one. Likewise, Mega Sharpedo is pretty cool in its own right, and I'd be more than happy to support a small raise based on how it can function in its own right. However, there's also the question of whether or not you'd want to use up your Mega slot for it, and I'm just not sure that the perks it offers are worth giving up the ability to use one of many other great Megas in OU more often than not.