Ok Mega Scizor is definitely one of the best mons in A+ rank right now. As you and others and said its a very potent blanket check on the meta, countering megagross, and is a reliable check (not counter) to the other s rank mons in clefable and mega altaria. Its move pool and power and defenses are great for the speed tier its in, and it has a very powerful priority move thanks to technician. And because scizor has an easy time setting up once its checks are gone bullet punch can easily clean through teams as it usually ends up getting multiple Sds thanks to its amazing bulk. However, it doesnt have the few final tools it needs to be S rank, using the other S rank as comparisons, and its push for S rank is being exaggerated.
Mega Scizor cannot get by a majority of its roadblocks like the other S rank mons can. Bulky chomp is either the most common mon or the second most common mon in OU, and it commonly runs fire blast in its free moveslot alongside stealth rock EQ and dragon tail. Scizor is pretty much walled by it to begin with and will get either worn down by it or get fire blasted. Mega gross is another mon that struggles with bulky chomp, but it has ice punch in its arsenal. Other notable counters that will in fact cripple scizor or knock it out are some of our OU fire types, such as char x( who uses it as setup fodder and is pretty significant to note) char y and talonflame. Now as u said these threats are somewhat "easy" to account for in supporting scizor because they all share a rather nasty weakness to Stealth rock, which is a valid point. however, these mons are still a stagnating presence for scizor and are rather common. Scizor's effectiveness in checking the fairies is also mitigated in that specs magnezone and scarf can easily trap it and kill it, which is pretty commonly paired with fairy types, for good reason. Scizor can get by this threat with super power yes, but its kind of a 50/50 depending on your scizor spread and your opponents Magnezone spread. Scizor also has superpower for heatran, but at the same time heatran also has a few tricks up its sleeve for beating scizor, because it can easily speed creep it unless its max speed jolly scizor, and even then scarf heatran is a thing, although not as common as it used to be. Other notable answers to mega scizor are mega manetric and zapdos who both resist either one or both of its stabs and can easily threaten it back with their fire coverage, and scizor can do little about it. Keldeo is also another common threat, threatening it with scald, and specs hydro pump ohkoes, and quagsire and skarmory pretty much stops scizor all together, versus something like the S ranked megagross, who shares some of these checks can easily get by them given the correct move pool. And even so, mega scizor lacks immediate power because bullet punch is relatively weak without a boost despite a monsterous 150 attack stat, and that combined with its low speed makes it easy for aforementioned checks and counters to force it out despite how good its bulk is.
To be frank, as Living So Zambian mentioned earlier, mega scizor does have a good matchup against many teams right now because of its typing granting it the ability to a large blanket check, and some of its success are indeed attributed to poor team building. A build with megagross a lati twin, a check with scald other than keldeo, and something like clefable will lose to scizor if it lacks any appropriate lure, such as hidden power fire latios, because a team like that will not lose to only scizor but other common steel types as well, particularly sand rush excadrill who also sits in A+. So with that said Scizor should remain A+.