Just clearing some things up with the above post:
"Flying already has a good amount of coverage, and having access to Sky Blast means that Double Normal teams have lost their niche as a hard-hitting neutral type for blowing past timed stages."
Double Normal is still the superior neutral option for timed stages due to better multiplier and higher base Pokémon without investment. The timed Rayquaza competition last week, even if it didn't have any skyfall, would still have Double Normal near the top, above Sky Blast, but not as high as a Pixie Power team.
"If there's anything that holds MegaMence back, though, it's the lack of defensive support that Flying has right now. All the type really has going for it is Eject+ Lugia and Stabilize+ Staraptor."
This argument goes out the window the moment you realise you don't have to run three Flying-types alongside Mega Salamence. Have your Sky Blast suite and have your fourth Pokémon be something tailored to deal with whatever bad disruption is on the stage, such as Cresselia for a barrier jungle from a Flying-type, a Talonflame for a Grass-/Bug-type with lots of blocks, you get the picture. And you won't even lose too much out of having one mon not being Flying-type, save not getting the bonus from Sky Blast.
"Outside of those, there's nothing that really stands out, and whatever types it does cover, it faces competition from other Megas with better support options (Blaziken vs. Grass/Bug, Mewtwo vs. Fighting)."
Salamence destroys Blaziken teams in terms of how strong they are vs. Grass-/Bug-types. Having an effective two-turn mega evolution thanks to Mega Boost (which can be PSB Farmed to have up to a 90% rate on a three-match, and oh, it's PSB-farmable event is available at the moment), doesn't have an effect as RNG as Mega Blaziken, and can run a non Flying-type without losing too much unlike Blaziken who loses a lot when you don't have all Fire-types. It does not even need Sky Blast to laugh at Blaziken teams.
MMY's only advantage over Salamence is Main Stage advantage and the ability to run Block Smash+ and Barrier Bash+ (and even Mind Zap) without losing much consistency. Otherwise, it is eclipsed in terms of how fast it can reliably evolve, is eclipsed in raw power, and is eclipsed when it comes to Sunday Meowth due to its heavy reliance on Mega Boost Support, and the inability to reliably run eject strats like Mence can with Lugia/Blank Slot.
Mega Salamence holds up very well against its competition and even surpasses them in some regards. The only major issue is that it comes at stage 530.