Zard-X is a bit of an oddball, tbh. I'm very much in agreement with the notion that it's one of the strongest overall attackers in the metagame w/ Tough Claws + base 130 attack in conjunction w/ a solid arsenal of moves to use (flare blitz, dragon claw, dragon dance, eq, roost, etc.).
The thing that really holds it back in my eyes is the fact that Zard-X isn't particularly easy to slap onto a team nowadays. If you think about it in perspective, the following factors hold Zard-X back in this regard:
- Weakness to SR (4x before mega, 2x after mega)
- Doesn't provide much defensive presence (you don't often see yourself saying "oh hey, my switch-in to "x" is my ZardX, unless it's the bulkier wisp variant which is fine and all, but not the threatening one or the set that would really bump it up to A+)
- Requires a fair amount of support (it's not Volcarona bad in this regard, but it still requires hazard removal of some sort, for sure, and it appreciates either a lure to Tran (Surf Latios, HP Ground Serp, etc.) or dropping roost for EQ (which limits it a lot) while it also enjoys something to weaken LandoT, Slowbro, and even Azumarill)
I think the best way to prove that it's not quite worthy of A+ besides noting the fact that it's simply hard to use is putting things in perspective, comparing its viability to other megas in A+/A.
The following megas are in A+: Diancie, Lopunny, Medicham, and Scizor. The following megas are in A: Charizard-X, Heracross, and Sableye.
If you look at the A+ megas, you can see that there is a fair variety among them. However, they all are, in my opinion, top tier threats and deserving of being ranked in A+, higher than Zard-X. Mega-Diancie doesn't have much of a defensive presence, don't get me wrong, but it does provide teams with the ability to keep hazards off the field or at lease ease some pressure onto the opponent when it comes to their ability to set them. Add this to the fact that, off the bat, Diancie can threaten almost the whole tier and the fact that, after protect, it's quite fast (110) and you can see why Mega Diancie is comfortably a top tier threat. I'd say that with the 110 speed, the mixed breaking prowess, and the niche Diancie has (ability+typing), it is much easier to use than Zard-X and offers more utility than it, too. Mega Lopunny is seen as a really good measure against offensive teams, especially w/ ice punch to hit landot/chomp, but in general because it's fast and strong w/ prio fake out and sometimes quick attack. Given that it's so fast, revenge killing abilities that Lop has almost compensate for the lack of raw defensive presence, especially because it can get in and mega fairly easily w/ fake out, so given how convenient it is to use and how it makes up for a lot of its shortcomings in some way or another, it's also safe to say Lop is a top tier threat. You'll see Lop very often in the current metagame and it really doesn't requite any support, either, so I'd say that it comfortably outclasses Zard-X overall w/ the speed, sheer convenience, lack of need for support, etc. all going for it. Mega Medicham is pretty much impossible to counter, or at least close to it, with the only form of consistent counterplay being Mega Sab, Mew, Reuniclus, bold Clef, and Slowbro (assuming it lacks TPunch) and even then, all of these things have to stay healthy as they're going to take 40% or so from HJK, bar Sab whose immune ofc. Fake Out allows for a quick, convenient mega and it doesn't really need much support. Basically, Mega Medi hits so hard that it's hard for it not to be A+ and while something like Zard-X is quite strong as well, it lacks that strong, non-recoil STAB like HJK that Cham has and it has many more drawbacks, making Cham a top tier A+ thread and Zard-X a step or two behind. Lastly, Scizor is totally different than all of the others because it's primarily used for its defensive presence nowadays, seeing as it walls a significant portion of the metagame without sacrificing momentum and even with posing a threat as a bulky SD sweeper. Alternatively, it can be an offensive thread w/ the SD 3a set, but that's not the main reason it's A+. My big thing behind mega Scizor is that it can be slapped onto teams and work as a way to check fairies, dragons, etc. while it just sponges so many hits thanks to the bulk and u-turn to gain momentum right after, if all works out. Zard-X and mega scizor are in no ways comparable, but scizor is by far more common and seen as one of the best pokemon out there period right now, so I think that's enough to justify it being A+ while Zard's not in the picture.
Overall, Zard-X has the ability to break through and sweep teams with proper circumstances, but it needs that element of support and it is hard to fit into teams, so it falls behind these more convenient and off-the-bat effective pokemon. Something like Heracross, which has its mega in A, doesn't need much support, but it has a different shortcoming in terms of it being slow and, therefore, relegated mainly to be used as a breaker of stall/balance because it, too, is hard to counter and can still outpace the slower pokemon out there. Mega Sab is a unique case in general, so I won't discuss it in comparison here, but generally, A rank megas are a step below the top tier, A+ rank megas solely because they're harder or more situational to use despite still providing a lot of upside. Zard-X would be A+ or even S if it was solely based on the upside it presents - how strong it is and what it can do w/ that strength and the moves it has - but this isn't the full picture and, therefore Zard-X should be A, imo.